Garageband (the software, not the site) users are starting to warm up to soundfonts thanks to applications like Andy Drabble’s Soundfont Synth which sells for $15 (although it sounds like there may be issues with version 1.0).
The site claims that it works in “any” AU host which presumably covers the upcoming release of Live 4.0 with it’s MIDI support which will open up whole new worlds for Live users. (“Will you be having some Grain Delay with your Steinway?”)
Soundfont support on the Mac is important because it spreads the ubiquity of the popular, yes “open” thanks to E/MU and Creative, widely used SF2 format (2.1 spec (PDF)). Having said that I’m not exactly eager to be first to try any of Andy’s stuff. Good luck!