Ok, so I know a lot of you are looking for a simple process to get the animation screen changed on non-hero/htc builds. Well, we can do it.
First, animated gif images dont work for google builds as of yet (at least that I've heard of), so scrap that idea right now.
The google boot animation is a combination of two images:
android-logo-mask.png

and
android-logo-shine.png

(images taken from Dude's 1.3RC1)
Now, the way this works is as so:
android-logo-shine is the color in the background. it scrolls across repeatedly, and that white stripe on there makes it look like it's got a light glimmer on it. android-logo-mask is the actual image. When it's all put together, it looks something like this (imagine the background color scrolling across as your phone is loading)
now, I've played with this some and my current logo is 256x265. The logo cant really be any wider than 256 pixels or it will crop off the edges of the screen. The shine has to match the image in height, but the width is fine at 512.
there are multiple methods for doing this. first is the old-school method for people who like to do things the old fashioned way.
So heres what you do: (
VIDEO TUTORIAL HERE: http://www.youtube.com/watch?v=QMrn3n_ayHI)
1.)take your favorite rom and favorite unzip program (i use winrar). open the rom up and navigate as follows:
system--->framework
2.)you will see a file called framework-res.apk. open this file (again, winrar works good for this).
3.)open the assets folder, then open the images folder.
4.)you will see the two png files i mentioned above. extract them both to your desktop, but DO NOT CLOSE THE FILE BROWSER (unless you want to navigate to that folder all over again).
5.)now comes the fun part: editing your images. this is where it gets a little more complex:
-on the mask, any area you want the scrolling color to show up must be TRANSPARENT. You can put just about any text or image on there that you want, but the scrolling color from the shine image will not show through without the transparency.
-the shine image can be any colors you want, so feel free to play around with some schemes until you come up with something you like.
6.)after you're done and you've saved both images the way you want, put them back in the same folder you took them out of (framework-res.apk\assets\images)
7.)once you've put them back in, you have to resign the whole update. this is a very easy process that is laid out for you in
this thread that stericson put together.
8.)throw the newly signed update.zip on your sd card, boot into recovery mode, flash, and reboot. you'll be happy with the result.
Just to give you guys a little taste, heres what my FIRST design looked like:
(just use your imagination to picture the red/white background scrolling across the android and lettering)

A couple things to point out for you artists: the "shorty's android" and the android outline are the only transparent parts, so they will show the scrolling color. The "1.5 customized" and android eyes are solid colors, so they dont scroll/change/nothing.
Now, at this point in time, I havent found any way of adding music like the htc/hero builds can do. If anyone figures that out, let me know and I can add it to this tutorial, with credit to you, of course.
Enjoy your spiffy new boot animations guys, and please post some screens here so we can all admire your work.
New addition guys: for those having trouble with the directions, I made a video tutorial to help out! Check it out here:
http://www.youtube.com/watch?v=QMrn3n_ayHI
Next, and probably the easiest method of doing this is to use METAMORPH! If you dont know what metamorph is, well, you're behind the times and need to check out this thread:
http://forum.xda-developers.com/showthread.php?t=591329
If you are familiar with using metamorph to at least install themes on your device, then you're set.
So how do you do it using metamorph? Simple.
First, you're going to create a brand new folder and title it "bootanimation" or whatever you choose.
Now, inside that folder, you're going to have several files in order for this to work right.
1. The folder stack where the boot animation will go (explained below)
2. the .thm OR .xml file depending on which version of metamorph you have. If you're not sure: free version uses .thm files. Pro/Donate version uses xml.
Now, creating these things is SIMPLE. The folder stack is going to be the same every time for boot animations. In your folder that you have already created and should be staring lovingly at, create a new folder called "framework-res". THIS FOLDER NAME MUST BE EXACT, as must all the ones that follow. open the folder you just made and create a new one inside it called "assets". Open the assets folder and create a new folder inside it called "images". I know it seems tedious, but thats the LAST folder you have to create, I promise. The last step here is to open the images folder and place inside it your android-logo-mask.png and android-logo-shine.png files. after you've done that, move back out to the original folder and get ready to create your .thm or .xml file.
Now, .thm files for bootanimations are simple. open notepad and put in the following:
any
any
framework-res.apk
/system/framework/
Then save the file (NOT AS A .TXT FILE) as bootanimation.thm
Now, when saving the file as indicated above, the name of the .thm file MUST be the same as the name of the folder you originally created. So if your folder, and thus your final zip file, is named purplebuttmonkey, then your .thm file must be named purplebuttmonkey.thm. Simple enough?
Now, creating the xml files is a bit more complex for the average user and certainly a bit much to put into text, so I'll simply defer you to the master, Stericson himself, in a nifty video he made to educate you how to do it:
http://www.youtube.com/watch?v=Mq4AuHKzNFo
So fast forwarding:
Once you've created your folder stack, your .thm file or .xml file and are ready to move on, all thats left to do is to create your zip file! Guess what, these dont have to be signed either! Hooray! So open your favorite zip program (again, I recommend winrar) and create a zip file out of the original folder you made. After that, put it on your phone, load up metamorph and go to town!
Also, metamorph is nice because it eliminates having to re-sign everything every time you want a new animation. All you have to do is replace the two png files in the folder stack you made, re-zip it and morph it once its on your phone.
NEW:

Hitchhiker's Guide To the Galaxy Boot Animation:
http://www.4shared.com/file/XWXh6pT6/HHGBoot.html
YOU MUST HAVE METAMORPH. For right now (4/25), only the pro version will work. Stericson is updating the free version to XML on the 30th. After that, this will work for everyone!
NEW ADDITION: Burlyskink made an excellent html applet for testing out your designs so you no longer have to flash an update in order to see what the finished product looks like! You can find this little gem
here. Many thanks to Burlyskink for this!