Windows 7 - Tip thanks to Pete Marshall
Here's a tip that may be useful for those with several drives wanting to speed up LR a tad.
1. Download and install Link ShellExtension if you havn't already
2. Move previews folder to new location
3 Right click on new folder select Pick Link Source
4. Return to catalog folder and right click in the folder
5. Select drop as symbolic link
You are now ready to go with previews on a separate drive.
Mac OS X - Thanks to Sean McCormack
Seans approach uses Symbolic Links. Further details on how to go about making the symbolic links and moving the previews can be found in his tutorial Moving the Preview folder