Mac OS X,  Mac OS X Server,  Time Machine

Mac OS X 10.5: Time Machine Config

You can customize what Time Machine does not back up by using the following plist:

/System/Library/CoreServices/backupd.bundle/Contents/Resources/StdExclusions.plist

Simply add the strings that you don’t want to back up and it will no longer back up those locations.  Remove the strings to re-add them at a later date.
In the UserPathsExcluded key, you can exclude paths that in relation to users home directories.