Snow Leopard uses 64 bit applications throughout the system. This is good for everyone, but sometimes older 32 bit code cannot work hand in hand with newer 64 bit code.
An example of this is System Preference panes. If you try to use an older 32 bit preference panel, System Preferences will quit and re-launch in 32 bit mode.
The next time you launch System Preferences, it will reset to 64 bit so you’ll be re-launching System Preferences often until those older preference panes are re-written.
This compatibility issue applies to all plugins, such as Mail.app plugins, when the host application is delivered as a 64 bit version.
Steve
P.S. Original Mac Book Pros, iMacs and minis shipped in 2006 are 32 bit computers and won’t run into this issue as they cannot use the faster 64 bit code