I no longer want to link my default Google Account on my phone. I have a Nexus One runnin 2.2.1.
How can I unlink my Google account?
Set up a new gmail account (you have to have one associated with the phone). Next, on your phone go to settings > applications > manage applications > gmail and choose "clear all data" do this for gtalk and google voice too if you have them set up. This should prompt you to start the account setup on the phone where you'll enter your new gmail acct info.
A method similar to the one that Matt mentions worked in pre-2.0 versions of android, all you had to do was hit the Clear Data button for the Google Apps application in Settings->Applications->Manage Applications
Since 2.0 and the improvements to account management that came with it, this trick no longer works. To the best of my knowledge (G2, myTouch Slide and G1 with Cyanogenmod 6) a master reset is now required.
If you need to backup info, try searching appbrain.com for share my apps, sms backup+ or sms backup and restore, and/or MyBackup Pro
If the "clear all data" trick (as described by Matt) does not work (it didn't with my HTC Magic/Android 1.6) you can change the password to the google account (from a computer) and reboot the phone. Sometimes it will ask you for your account info right away after booting and sometimes you have to access some google app like the calendar or mail to trigger the account questionaire.