How to back up the address book?
Your contacts are automatically backed up to the Google account you set up your phone with.  So long as you have the "Contacts" sync setting on, any changes you've made to your contacts should be mirrored in your Gmail contacts online.  After you wipe your phone, if you use the same account to set it up again the contacts will automatically download on the first sync.

You can verify that your contacts are backed up by going online to Gmail, clicking on "Contacts", and checking them against your phone's address book.