The largest increase in demand for iPhone developers came shortly after the Apple App Store opened, which no doubt drew both 3rd party companies and independent developers in. What's even more interesting is what some people see this leading up to, which includes accreditation for development. Could we see in the near future a "Certified iPhone Developer" program come out?
With a massive increase in developers, they also expect to see an increase in conflicts over intellectual property. This could easily happen between different developers, or between the developers and Apple. Apple has already proven they are willing to squelch any application they don't approve of on the App Store, which has included programs they feel "mimic" existing ones that they develop. Regardless, iPhone development has seen a huge surge, which is nothing but good news for the iPhone.