I never thought I'd say that, but Apple's making me wonder. Apple says 100,000 developers have already downloaded the iPhone SDK (link). For comparison, it took Palm a couple of years of heavy evangelism to hit the same milestone. That's a deceptive comparison, though -- Palm was a small and relatively unknown company at the time, whereas Apple is a huge brand, with a large base of current Mac developers that it can bring over to the iPhone.
So the process was quite a bit easier for Apple. But still, criminey, 100k is a lot of SDK downloads in just four days. Apple has clearly struck a nerve.
In addition to the 100k statistic, the Apple press release included new endorsements from companies like Intuit, Namco, PopCap (Bejeweled, baby), and SixApart. It's a very interesting variety of companies.
Which raises an interesting question: How in the world will all those developers find an audience? Based on the current iPhone installed base, Apple already has one developer for about every 40 iPhone users. Let's be optimistic and say every user will spend an average of $20 a year on applications (a figure much higher than we've seen on any other mobile platform). Apple keeps 30%, so there will be a total of $560 in revenue per year available to each iPhone developer (not to each app, to each developer). The iPod Touch will increase that number a bit, but not enough to change the math. Either Apple has to grow the installed base enormously, or there are going to be a lot of iPhone developers going hungry.