There are almost certainly not 600,000 paying iPhone developers. I'm guessing that number is people who have downloaded the SDK (which is available for free).

There are only about 35,000 apps, and the average number of apps per developer (who has an app on the store at all) is probably closer to 2 than 1. Even with twice as many paid developers as people who have shipped apps, you're talking about a maximum of 75,000 devs.

That also means that the mean revenue / developer is $300... Doesn't sound like too profitable a business as $99 of that $300 has bought you the registration fee ;)