I was looking for a way to have multiple audio inputs into GarageBand, say using my Digital Line in, two iMics, and anything else that will get audio, but GarageBand only lets you choose one input at a time. But with a tip from the guys at Rogue Amoeba, there is a way!
Using the Audio MIDI Setup application (in Utilities), choose "Open Aggregate Device Editor" in the Audio menu. Here you can create a virtual device that combines all your other inputs. Click the + sign to create a new device, then check "Use" off on the devices in the list that you want in your virtual device. If you like, double-click "Aggregate Device" to change the name to something of your choosing.
In GarageBand, choose your new device from the input list and there you go! Theoretically, you can buy four iMics and an USB hub and you can have eight inputs -- but I don't know the maximum number of audio streams that can be used simultaneously over one USB channel. But, this is a lot cheaper than buying an eight-input device which can run up to $800. Enjoy!
This made my day!