Contrary to what's been reported on this page, GarageBand 2.0.1 has added direct support for hardware controllers. Select the GarageBand application, do a "Show Package Contents" and you will find a "MIDI Device Plug-ins" folder. Inside are bundles of control interfaces for two devices. The first is the M-Audio iControl, which was announced at the Frankfurt Musikmesse last month but which has not yet been released. It provides transport control buttons, a jog wheel, and lots of buttons for controlling track functions, mixer volumes, and even real-time effects parameters.
The second control surface bundle is called "Rascal.bundle". Nobody I have talked to seems to know what a "Rascal" is. Obviously it is another as-yet-unreleased hardware controller from a third party.
But here's the big news. Controller surface support plug-in bundles from Logic Express 7.0.1 will work in GarageBand. This is undocumented and I doubt Apple wants this to be known.
If you have Logic Express, you can select the application and do a "Show Package Contents". Again, inside you'll find another "MIDI Device Plug-ins" folder. You can copy the four bundles that ship with Logic Express and paste them into the analogous folder in GarageBand, and they work.
By doing this you can add support for the Tascam US-428, Tascam US-224 (through its "428 Emulation Mode"), Tascam/Frontier FireWire 1884, as well as Logic Control and SI-24.
I have tested this with the Tascam US-224. I can use the US-224 to control GarageBand play, record, stop, rewind, and fast-forward, and also use mute and solo buttons and real-time volume faders for any group of four contiguous tracks, as well as master volume.
Unfortunately I can find no support for the jog wheel, or the locate buttons. GarageBand does not have any locate marker functions in the software anyway.
Let's hope Apple continues to expand the third-party hardware controller support in Logic Express and to make it available to GarageBand users. Proprietary lock-in is not a good thing.