I'd keep it simple for search engine purposes...
A Blackberry site is a Blackberry site. An iPhone site is an iPhone site, a Windows Mobile site is a Windows Mobile site.
I vote no...not because of any particular allegence to a brand. I'd likely have gone with an iPhone had it been available on VZW - AT&T just doesn't cut it in my neck of the woods.
It just seems simpler to have seperate sites to search/drill down to the needed information.
At this stage of the game, what could an iPhone user expect to find in such a site? As I understand it, one can't even change the battery, let alone mess with an operating system. So, other than selling iPhone applications, I see no real benefit to comingling the communities/forums.