Short answer, from my understanding, yes and no
In a bit more detail, there's nothing stopping you from writing an iPhone app and publishing the source under GPL. However, certain parts of the license seem impossible to fulfill under the app store distribution mechanism
The real-world result is situations like Colloquy mobile, which you must pay for on the app store, but can get the source for free of charge (and could build/install onto your iPhone/iTouch if you're signed up for the $99 dev program)
