I haven't asked Chris, but my guess is that it's because it would become too cumbersome over time to have a separate case forum for each and every iPhone model that comes out. This is particularly true now since the iPhone 3G cases will fit the iPhone 3G S, and it wouldn't make sense to have a separate case forum for these 2 devices. I'll flag this for Chris, though and will let you know if there's a different answer...
UPDATE: Well,
looks like I was wrong.

The 3G cases forum is now open.