Actually, you can use the standard UNIX interpreter syntax directly with osascript; just mark the file as executable and you're set.

ensure_mate.scpt

#!/usr/bin/osascript
-- invoke with $ osascript ensure_mate.scpt /path/to/project
-- or create a wrapper script that looks like this:
--    #!/bin/sh
--    osascript ~/bin/ensure_mate.scpt $@ &
-- Created 2010 Dan MacKinlay and hereby released into the public domain

on run (wantedPath)
	set wantedPathFound to false
	global wantedDoc
	
	tell application "TextMate"
		set allDocs to documents
		repeat with doc in allDocs
			if path of doc exists then -- beware unsaved files!
				set docPath to path of doc
				-- "starts with" because we assume we are opening a folder
				if docPath starts with wantedPath then
					set wantedPathFound to true
					set wantedDoc to doc
				end if
			end if
		end repeat
		if wantedPathFound then
			-- this branch should bring the folder window to the front if already open
			-- doesn't work - window has no document, for some reason
			-- see http://lists.macromates.com/textmate/2009-March/028352.html
			local currDoc
			
			repeat with wind in (get every window)
				if document of wind exists then
					set currDoc to (get first document of wind)
					if currDoc is wantedDoc then
						set index of wind to 1
					end if
				end if
			end repeat
		else
			-- elsewise just open it
			open wantedPath
		end if
		
	end tell
end run