Awhile back I seem to remember Allen suggesting he could create a TextMate protocol (e.g. txmt://path/to/my/file). I've come to the conclusion that if this was available, I would use it a lot.
Here's one example:
I use Markdown syntax for most of my writing nowadays, but once I've written something, I want to view it looking nice in a web browser. Invariably, I need to edit the file later. I have three options: I could put up a text area and edit it in the web browser. I could launch my ftp program and Edit Externally... the file. Or I could automatically launch TextMate, my editor of choice, with a simple txmt:// link on the web page.
Best, Eric