Dent du Midi is a freeware utilty to convert standard MIDI files for use in GarageBand.
[robg adds: I haven't tested this one, but I've received a few emails asking this question, so here's (apparently) at least one answer...]
Update: This hint is not necessary if you have iLife05 -- GarageBand2 includes native MIDI support.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040127031557246