You can always command-click the window title. However I agree it would be nice to see the filepath, maybe make this an option in the preferences.

Until then, you can always set up a key command, here's what I did (using growlnotify)

/usr/local/bin/growlnotify -n "/Applications/TextMate" -I "/ Applications/TextMate.app" -t "$TM_FILEPATH" -m "Is the current path for $TM_FILENAME"