PBee,
There are 2 options available to you:
1. This requires that you registered an alternate email address when you setup your GoogleMail account
i) Goto
www.gmail.com
ii) Click on the 'I cannot access my account link)
iii) Select forgotten username and follow the instructions
2. This will wipe all data from the G1 and return it to the factory setup (use with caution)
i) Turn off phone
ii) Turn on phone whilst holding down the "menu" + "home" keys, keep them pressed until the triangle logo appears on the screen
iii) Press "Alt" + "W"
iV) Phone will reset its software to "out of the box" settings and reboot