⇧⌘. shows hidden files in file dialogs and in some third party applications like TotalFinder. Transmit also has a menu item for toggling hidden files, but it's assigned to ⇧⌘B by default.
There's nothing equivalent to that in Finder though. But you could assign a shortcut to a script like this with FastScripts or by creating an Automator service.
k="com.apple.finder AppleShowAllFiles"
[[ "$(defaults read $k)" == 0 ]] && b=true || b=false
defaults write $k -bool $b
osascript -e 'quit app "Finder"' && open -a Finder
