If you sign up for a 60-day .Mac trial, which is completely free and doesn't require a credit card. When the 60 days expires, your iDisk, email, syncing, and Back To My Mac all expire with it, however it leaves your
xxxxxxx@mac.com address usable for iChat forever. You can follow vanmacguy's instructions otherwise. That way you can choose your iChat account to be xxxxxxxxxx or
xxxxxxxxxxx@mac.com. The nice thing about going with the .Mac username is that it is really hard to get what you want for your AIM username, because everything is taken. Good luck though!
Edit: Go to
https://www.mac.com/WebObjects/Signup.woa/wa/trial?aff=consumer&cty=US&lang=en to sign up for a free .Mac trial. If it asks you to enter your credit card number, you've gone too far.