Is there a way to use a Finder hot key to open a folder in TextMate?
I think that's a bug with Finder. Services that receive folders as input don't seem to be listed in the Services menu when folders are selected (but it only applies to column view; they are listed in other view modes).

In any case, one alternative would be to just use an AppleScript without wrapping it as a service. You can give it an app-specific shortcut with FastScripts.

try
    tell application "Finder"
        open (get selection) using path to application "TextMate"
    end tell
end try