Roger,
I haven't been able to verify it directly but I have found that there is software out there to allow MIDI commands to control GarageBand. That seems to me to indicate pretty strongly that GarageBand won't natively respond to MIDI and that you were only able to see it happening because of the iControl.
I've found Garage Remote that will allow the control of the transport controls.
http://www.versiontracker.com/dyn/moreinfo/macos/23026
Even more interesting is MidiPipe which will do very many things with MIDI data. In relation to GarageBand it will listen for MIDI messages and then fire off AppleScript programs in response to the MIDI. That will let you do virtually anything with MIDI messages on the Mac. You could use it to control GarageBand, Safari, Mail, anything.
http://www.versiontracker.com/dyn/moreinfo/macos/16154