ControllerMate - Rock Band guitar (detail)
Here's (a very small part of) how I have my Rock Band guitar hooked up to GarageBand on my Mac, using ControllerMate.
It turns out to be surprisingly complicated to emulate the logic. Holding the green button and strumming creates an emulated 'a' keypress, which is held until the green button is released. Additional strums while green is held do what you'd expect.
(Inspired by Ken's post at www.orderedbytes.com/forum/viewtopic.php?t=701 and a slight improvement on that version, which seemed to trigger the sound on a strum even without the fret button being pressed)