Look on the market for "YouTube Ripper" - it's a free application - requirements are: A SD Card, Android 1.5 or better.
After you installed you can use it like this:
1. Open YouTube application
2. Search for your favorite youtube video.
3. Press on it until a menu pops up, press on "Share"
4. Select "YouTube Ripper"
5. YouTube Ripper application pops up, press on "Rip It!"
It will then start downloading the youtube video to your /sdcard/youtube folder.
. Rewritten from scratch
. Fixed a bug that would a occur if you'd download more than one file at a time.
. Improved filename correction code
. Downloads will now show up in the notification bar
. Progressdialog can be cancel'd
. Improved videoId detection code
. Enforcing portrait view
. New Icon - thanks to rori~!
. Support for Android 1.5+
. Added a download information layout
. Files now be saved to /sdcard/youtube
. Uses YouTube Title for the filename instead of the id.
. Added Adds to get the app into the market \o/
. Initial Release