(The best solution I found so far consists in creating a new project, adding the directory from which I want to do the search, and running a "Find in Project" in that new project. But I have to say that this is quite clunky.)
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The solution I use is to create a temporary "sub project" like you mention, only my method of creating the sub project is quite simple: drag the folder you want to search from the drawer to the TextMate icon in the dock.
Dru