Where are keyboard shortcuts stored (for backup and sync purposes)?
The shortcuts that can be set in System Preferences > Keyboard > Shortcuts > App Shortcuts are stored in ~/Library/Preferences/.GlobalPreferences.plist and the property lists of applications (like ~/Library/Preferences/com.apple.iTunes.plist or ~/Library/Containers/com.apple.chess/Data/Library/Preferences/com.apple.chess.plist).

The shortcuts for services are stored in ~/Library/Preferences/pbs.plist.