MIDI hepcats understand the benefits of Propellerhead Software's technology (a technology that allows two applications to share audio data). The recent hints that good things are coming in regard to ReWire support.
When you launch GarageBand 1.0.1 and then Propellerhead's too-cool-for-words-music-station-software, Reason, you'll discover that the two programs talk to one another. Reason enters slave mode and when you press the Spacebar or Play button, both Reason and GarageBand begin playing. Any master volume settings you've created in GarageBand also apply to a running Reason sequence.
A tiny gotcha: If you also have installed, you won't hear your Reason tracks. If you move Live's support files (do a search under "Ableton" and move anything you find), log-out, log back in again, and launch GarageBand and then Reason, the Reason tracks play as they should.