Stackoverflow is supposedly one of the more popular Q&A sites for IPhone developers:

http://blog.stackoverflow.com/2009/06/podcast-57/

...Stack Overflow has become a very popular destination for iPhone development. This is completely accidental, but it is a valid reflection of the vibrant and growing iPhone development community. If you’re an iPhone developer, check out the Mobile Orchard website and podcast, which even has a best of Stack Overflow for iPhone developers!...