Sorry I missed this post before - for some reason, it didn't show up on my page 1. Mitchell Sigman and I worked together on the MainStage section of that massive review, so I'll ask him what he used and report. I've used MainStage with a few different MIDI keyboards: Studiologic VMK-176 Plus, M-Audio Keystation 88es and Axiom 49, and Edirol PCR-M1 and PCR-800. Mainstage is good enough at handling everything on the receiving end that you almost don't have to worry about how your controller keyboards are set up, though as a matter of good housekeeping.
If you're OK using the 1/8" onboard output on a Mac laptop, which I did with satisfactorily low latency at a gig where I forgot the power supply for an audio interface (current in the venue was low so USB bus power was unstable), then you've got all you need. For the best audio quality, I'd recommend using an audio interface. Lately, I've been playing around with the Apogee Duet, M-Audio Fast Track Ultra, and Novation NIO, all of which are up for review soon. Actually, the Duet review will be out in the April issue, which a few subscribers may have already.