I find these two articles interesting reading. One is a very positive uplifting article regarding the opportunities writing iPhone apps (written by someone who has had a #1 ranked game and is obviously making decent money) and the second is by Newsweek, putting a more somber tone on the idea of trying to make money in the Apple App Store.
http://criticalthoughtgames.com/node/109
http://www.newsweek.com/id/216788/output/print
My personal opinion? Both articles make valid points. I'm not expecting to become a millionaire via the iPhone. I originally quit my job over a year ago to pursue a more challenging career and try to make it on my own doing freelance development. The iPhone is just one platform I was going to use. As it turned out, iPhone development turned out to be very interesting and enjoyable platform to use (even with all the frustrations the come with having to deal with Apple's review process) and I've been able to stay afloat financially concentrating solely on it for the past year. But, when I say stay afloat - I mean just that. I'm making about 1/4 of the salary I was making as a senior software engineer at a corporation. My revenue now is pretty much equal to my expenses - my bank account is about the same as it was when I first quit my job a year ago.
Looking out a little more long term, I don't think I'll continue to focus primarily on the iPhone. The income I am getting is just barely meeting expenses and it doesn't make financial sense to continue the sole iPhone focus indefinitely. At some point I will start working on other platforms to see if something else is a bit more financially viable. I'm not looking to get rich, but it would be best to do more than break even on the expenses.
I definitely have no regrets about quitting my job, though. The change in lifestyle gained from being my own boss and running my own business is well worth the loss in income. It's hard to put a price on freedom - and I suppose I should be grateful for the iPhone, since it has given me my start with this freedom.