If you are not aware of this new implementation and awareness in Snow Leopard....
Some AU plugins have been able to work with Leopard 10.5.x if the developer has not updated them for Leopard or Logic Pro 8.0.2 or 9 compatibility and you can force them to validate, although the Compatibility column in the AU Manager for Logic Pro 8.0.2 or 9 will state "failed validation", they will appear and may still work in the Incompatible AU folder located at Inserts > AU Instruments and I/O > AU Instruments. My point is that if Logic Pro 9 is used with Snow Leopard 10.6 that the AU Manager plus Snow Leopard incompatible software awareness and implementation during install or migration to Snow Leopard known-incompatible software will be moved to a folder named Incompatible Software on your hard drive. "Snow Leopard also prevents known-incompatible software from opening. If you see an "Incompatible software" message, contact the software's vendor or visit their website for a later, compatible version."
Although the Snow Leopard incompatible software awareness and implementation may be at application level some AU plugins require the Stand Alone Application to be installed into the correct folder during the use of the AU in the host DAW and this will affect the AU plugin from working correctly or at all whether or not you can even can force the AU plugin to validate with any AU Manager.
**Mac OS X v10.6: About incompatible software**
http://support.apple.com/kb/HT3258