If you make a forum, please don't spam it with 100s of different android device subforums. I could scream, when I see an android forum a mile long with images of those android devices.
I don't think, there is a need for an iphone forum, because that market is already served.
For the sake of completeness you can make one, but I wouldn't expect much traffic there
I would rather advice to pick a niche and try to make it as good as possible. If you want to serve all purposes you will end up not having much time for anything and that will result in a poor experience. I know what I am talking about, believe me
What I would do (to stay on topic) a really good android review site, updatet more than once daily and a forum, which is concentrating on apps and developer needs. Nothing else. No secondary market, no Winphone, no hacking and rooting and such. You wouldn't get too much users probably, but you would get quality users.
I would make such a site if I would be at least a little bit interested in Android, but even if I already have a few apps on the market and even if I like my Samsung android phone, I still hate everything to do with the Android community.