I was receiving an error when starting up Garage Band, “… had detected a possible conflict between one or more third party MIDI or audio drivers.”
I checked several forums which indicated the problem had to do with an old driver version that was not compatible with Leopard (OSX 10.5). The only drivers I had in the HD/library/audio/midi drivers folder were ones from Digidesign, so I checked their web site, http://www.digidesign.com/index.cfm?navid=54&itemid=29552.
It appears that ProTools LE Midi drivers are not compatible with Leopard at the time of this writing. I created a backup folder and moved all my midi drivers to the backup directory. After moving the drivers out of the midi drivers folder, Garageband opened and functioned as normal.