I use QuickSilver (http://qsapp.com/) to 'Open With...' and it doesn't suffer from that problem.To set QS up to open your files from Finder, go to the Quicksilver 'Triggers' options and set a hotkey for 'Select Current Selection in command window'. Change from 'Custom Triggers' to 'Quicksilver' or 'All Triggers' to see it.
Select a file in Finder, press your hotkey, and QS should open with your file(s) as the target (first pane). Press 'tab' to go to the second QS pane (the 'verb'), type 'with' to change the verb to Open With... and press 'tab' again to move to the third pane (second target). Enter TextMate and press enter to open your files with TextMate.