I don't think the noise made by the iPhone speaker will be picked up by the mic. The phone cancels sounds generated by the speaker. I read this once, and if I find the source I'll post it. Empirically, though, you can tell this is the case when you use speaker phone. If the mic picked up sound from the speaker that's an inch away from it, the feedback would be terrible.
Having said that, the only sure way to see if it will work for your situation is to try it out.