Whatever you create, even if it's for iPhone first, don't get bogged down on it being on "platform X". Photoshop, as you mentioned, is a great example, it didn't need to be on a Mac to be great. It just was developed there first, and now sells more on Windows (I think).

If you focus too much on the platform, once that market wanes, you end up nowhere. This is what I meant by the MFC guy -- maybe an even better example is the VB 6 guy. We know this guy, right? He's an expert with VB 6. He made a damn good VB6 app 10 years ago, which he's supported since then. But he spent too long focused on VB6, so god help him trying to get a job today doing C#.

Your idea doesn't have to be as big as Facebook. All I'm saying is, don't get so focused on the platform that you become that VB6 guy. Focus now, sure, but developers over time should be broad and flexible. And if you are starting something new, your idea should be too.