Developing for the Mac or the iPhone
I have to say, I do really enjoy my iPhone. It's the best phone I've ever had, and a wonderful mobile computing platform to boot. But when I think about developing for it, I have to look at the stories of App Store Rejection, and wonder if it's worth it. Sure, more apps are accepted than rejected, but is that because they are little productivity apps and games? They can't really push the envelope, or they'll get rejected. Simple things like dictionaries, game emulators, and such are all being rejected by Apple on grounds that seem, at least to many, to be nearly arbitrary.
Coding is a lot of fun, and you have to be passionate about it to do a great job. This "maybe I will, maybe I won't" situation could have you invest a good chunk of time into an app, and then find that it's not allowed to be presented to potential users. Free or not, the same rules seem to apply.
I'm just not convinced that I could go through that cycle. Writing for the Mac, however, has no such limitations. This represents a clear choice to me.