As described over at “Mac OS X (Tiger and Leopard) Configuration Tips” you can configure Finder to display the full path to the currently displayed folder by using this (in a terminal of your choice):
defaults write com.apple.finder _FXShowPosixPathInTitle -bool YES
Change back to the default (of the default) using ‘NO’ instead of ‘YES’. Apparently this works in Leopard (or later) only.
