This applies to Android 1.6. If you need to switch your phone to a new Google account, follow these two steps:
Main drawer » Settings » Applications » Manage Apps » Google Apps » Clear Data
Main drawer » Settings » Data Synchronization
Once you get to Data Synchronization, it will prompt you to create a Google account or login to your existing account.
Warning: This will re-synch all your Google data… GMail, Calendar, and Contacts.