Apple has released Garageband 5.1 via software update (139 Mb for a Mac Intel).
This update addresses general compatibility issues, improves overall stability, and fixes a number of other minor issues. The issues addressed include:
GarageBand track effects and Audio Units can now be added to a guitar track
Improved support for Apogee audio interfaces
Faster switching to full screen in Magic GarageBand
Improved access to audio monitoring settings
This update is recommended for all users of GarageBand '09
