You can, but you'll need an adapter, here are some samples:
I tried using one of those cables from kvconnection. If i take the "control output" from my behringer mixer, the levels come into garage band ok on my ipad. The cable did seem to solve the problem where i would hear a repeated pop when i had both the line out and line in connected to the mixer at the same time.
But the issue i have now is that the audio seems to be losing some bass frequencies. The kick drum has no thump, etc. Am wondering if the attenuator on the cable is doing this or if something on the ipad or garage band is doing that.
Has anyone successfully recorded quality audio into your ipad using the audio input cable? (not using a digital interface)