The "beta-status" iPhone SDK, which was released just last week following a lot of coverage, has proven to be quite popular among developers. In the mere week it has been available, it has already netted over 100,000 downloads .
Initial demand was so great, in fact, that Apple was dealing with traffic issues on the first day of release. A lot of big-name software companies were mentioned in Apple's press event as looking forward to creating software for the increasingly-popular PDA, but they cannot come close to those numbers. Curious hackers aside, those numbers indicate that there might be an explosion of software coming out for the iPhone in the coming months and years. While Apple does require that developers pay to be an official developer, the $99 per year fee is pretty small compared to what many vendors charge.