Good question. Definitely not 500. Less than 100, but more than 25 I'd say. Guess = 45
Why not 500?
4000 developers ranging from newbies to full developers shot.
A couple of game engines are ready to build for iPhone. So you could see 50+ native games.
There are thousands of WebApps existing and you see 100+ of these translating to Native apps ready for day1.
Plus those that are just bundled WebApps like little javascript games. another 100+
Then there are the Jailbreak Apps who had a bit longer than the rest for development. They would have 50+ Apps ready for general use, not including those that aren't needed once you have app store.
Then there are productivity apps which could be low in number but still maybe 50-100, from companies that would have loved to do apps earlier but wouldn't touch Undocumented API's.
I think <200 would be low, 300-500 about right, 500+ impressive.