AppleInsider has noted that AT&T's website for existing users now has an option for a black iPhone.

There have been rumors that Apple's upcoming 3G iPhone would come with a glossy black back (1, 2). While there have also been some reported image leaks as well, the authenticity of such images remains highly doubtful.
Update: According to one report from the forum thread, Apple's online retail staff is indicating that news about the iPhone may be forthcoming in the next week. Whether this pertains to the current iPhone availability shortage or if it indicates a 3G iPhone announcement is uncertain. Typically retail staff are not told about upcoming announcements until immediately beforehand, however the iPhone shortage may have prompted a change from that procedure.
Update 2: An AT&T spokesman spoke to Gizmodo regarding the 'iPhone Black' option on their website.
An AT&T spokesperson just told us that the iPhone Black dropdown reference on the AT&T Wireless website is just a temporary placeholder. The spokesperson says that it was used over the weekend for a "scheduled catalog update," distinguishing the various iPhone models (4GB, 8GB and 16GB).
The explanation does not explain why the "black" distinguisher was used.