Don't follow the above just to change your gmail account! Although it is the perfect answer to resetting the entire phone. If you want to change your mail account without having to reinstall all your apps follow the instructions below that I found in another forum here. Sorry, I didn't catch the original poster's name.
First: your gonna need a google account that's set up with the proper "nickname" that you want (if you don't already have one).
Second: on your phone do the following at the home screen;
press menu ---> settings ---> applications ---> manage applications, you will get a message "Computing application sizes" then it will go to all apps installed on the phone.
Third: scroll down to "Google Apps" and click it to enter it.
Fouth: click on the "Clear data" button, you should get a black triangle with a white "exclamation point" (!) in it after a few seconds in the top left corner.
Fifth: click the back button until you get back to your home screen.
Lastly: pull down the notification menu and click on the message "Your account must be updated" Follow the onscreen instruction till you get to where it asks you to "create" or "sign in", click "sign in" and enter YOUR google account info. You'll get a message saying that you are signing in and it may take up to five minutes... then click "Finish"