According to Allan Odgaard:
This is not the case here -- I'm in TM now started from Mail, and TM was already running with several files open...
The service is unavailable if you haven't selected text, or in some applications, if the text selected is not editable, but other than that, it should be available.
Yes, after looking more into this, I've found that it varies from app to app. Mail.app has the option greyed out till I select something (which I found pretty annoying anyway).
Thanks and sorry for the false report.