I realize that between work and Cyburbia most people probably don't have a lot of time to frequent other message boards, but if you do which one(s)?
During my down time:
Cyburbia, Skyscraperpage and Skyscaperguy to satisfy my city-geekdom.
Plastic.com if I'm feeling slightly intellectual.
Fark.com if I'm not feeling so intellectual.
(Disclaimer- I don't actively participate in these forums I mostly lurk)