Thoughts from A Noob
First and foremost, thanks for these instructions. Since my 4GS is my only phone, I didn't want to risk trying the S-Off method and cooking my SD card. It seems to be almost risk free but I wasn't going to push my luck. Now running CM9, Alpha5 and loving it.
As a relative noob to rooting phones, I'd like to make a suggestion. I've read every post in this thread and if I missed it somewhere, my apologies.
I rooted my phone close to a month ago so I'm doing this from memory:
After extracting boot.img and placing it in the folder with the fastboot file, I ran the "fastboot flash boot boot.img" command and received an error stating something to the effect of needing some adb files. Like I said, I'm relatively new to rooting so I got stumped. I did some research and found out that I needed the Android SDK to get the missing adb files.
I already had SDK downloaded so I searched through the folder after extracting the files and again, no luck. Ran SDK, found that it needed to be updated which I did and voila, there were the adb files I was told were missing when I ran the fastboot command. I'm not sure which update needed to be run to get these files so I just did all of them. If my memory serves me right, the adb files were in the tools folder. Once I located the files, I moved them into the same folder where I had placed fastboot and boot.img, ran the command again and everything worked like what's described in your instructions.
I'm no stranger to computers so figuring out this needed to be done took some research and a bit of jumping around XDA threads before I put all the pieces together. For those other noobs who may not be as computer savvy, I can see this causing quite a bit of confusion. Unless I missed it, I have only seen one post that makes any mention of the adb files, which ones are needed, and including them in the same directory as fastboot and boot.img, and I didn't see anything in reference to needing the Android SDK (in this thread). Perhaps adding this info into your instructions would help to eliminate some questions and confusion and make it easier for others to flash the custom ROM of their choice if they don't want to try to S-Off method.
Again, thanks for these instructions.