You can find the current list of system exclusions at /System/Library/CoreServices/backupd.bundle/Contents/Resources/StdExclusions.plist It is an XML file you can open with any text editor.
Mine (Mac OS 10.5.8) looks like this:
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"><plist version="1.0"> <!-- paths we do not want to include in a system backup --> <string>/Desktop DB</string> <string>/Desktop DF</string> <string>/Previous Systems</string> <string>/Users/Shared/SC Info</string> <string>/private/var/db/Spotlight</string> <!-- old tiger location of the Spotlight db --> <string>/private/var/db/Spotlight-V100</string> <!-- old tiger location of the Spotlight db --> <!-- paths we need to include in backup so we can restore disk structure, but don't want to backup contents --> <!-- standard user paths we want to skip for each user (subpath relative to root of home directory) --> <string>Library/Application Support/SyncServices</string> <string>Library/Mail/Envelope Index</string>
