iPhones have terrible heat dispersion compared to even a fanless computer like a macbook air. You get a few minutes at full load before thermal throttling kicks in, so you could do the occasional build of your iPhone app on an iPhone but it'd be pretty terrible as a development platform.

At work we had some benchmarking suites that ran on physical devices and even with significant effort put into cooling them they spent more time sleeping waiting to cool off than actually running the benchmarks.