It’s not on Software Update yet, and it’s not prominently linked on the support site, but GarageBand 1.0.1 was released today.
It looks like they’re just clearing up the performance warning dialog boxes (many of which were either confusing or just plain wrong).
Update: Some folks are reporting other improvements, such as performance of some keyboards. I noticed that the online Help includes some new FAQs (and a working link to the keyboard shortcuts), but nothing else yet. The error dialogs are definitely clearer, though: