Gmail users:

• It’s not currently possible to change your Gmail username after you’ve registered. You can, however, create another Google Account with a different Gmail username.

Other users:

• If you don’t use Gmail with your account, you can change your account email address. However, it can’t be changed to an existing Gmail address. If you’d like to change it to a Gmail address, you’ll need to add Gmail service to your account.
• The account username that you choose needs to be an active, valid email address that you have access to.
• If you don’t have Gmail service on your account, the account username is the full email address you used to sign up for your account.
• To change this email address:
  • please visit the Google Accounts homepage and sign in to your account
  • Click the Change Email link and enter your new Google Account username.

Note: although you can’t change the email address on your Google Account to a Gmail address, Google invites you to add Gmail service to your Google Account.

(Info copied from Google Groups Help – 28th Jan 2008)