Using something like: "sudo open -a TextMate <file>" will not cause TM to run with super user privileges.
What will work, though, is using this fabulous program called Pseudo:
http://www.versiontracker.com/dyn/moreinfo/macosx/9608
With that you can launch any GUI application as root.
.Of course native support for this in TextMate is even better, but until then, you might want to give Pseudo a shot. -- David Heinemeier Hansson, http://www.rubyonrails.org/ -- Web-application framework for Ruby http://www.instiki.org/ -- A No-Step-Three Wiki in Ruby http://macromates.com/ -- TextMate: Code and markup editor for OS X http://www.basecamphq.com/ -- Web-based Project Management http://www.loudthinking.com/ -- Broadcasting Brain http://www.nextangle.com/ -- Development & Consulting Services