Hi Blair,
Alesis makes a dock for doing what you're describing. iPad fits in and the you get two mic inputs and some phone jacks for plugging in a guitar. Saw it at a music education conference and it did seem pretty slick, BUT it is really designed to be a replacement for those little "4 track studio in the bedroom"
I'm with JohnR, a laptop with a FireWire or USB mic preamp so you could record in WAV or Aiff would be what you want.