I'm sure everyone has their preference on how this should work, but I just wanted to point out that there's existing functionality in Quicksilver that could be used here (and it would be available system wide, not just in TextMate). With Quicksilver you could:
* Select your message text (in any application) ⌘⎋ ⇥ Compose (Email To…) ⇥ Type part of a recipient's name ↩
Or, if you were trying to populate a header line, like "To: ", you could:
* Invoke Quicksilver Search for a contact → Choose the right e-mail address ⇥ Paste ↩
I'm not sure if I'm making it sound complicated or not, but the process is pretty fast in practice.
Rob