To be absolutely clear, the thing I really think is critical is multiple documents. Multiple views of a single document is very useful, but I can survive without it (at least knowing that it's coming in the longer-run). But having a way to manage "windows" (panes in a single window being the way we're talking about) for multiple separate files in the same project -- and to switch the contents of each "window" quickly, with the keyboard (like cmd-T, or ctrl-x,b in emacs land) -- is absolutely critical.
And I'm not really sure where the "multi-project" idea came from -- I didn't mean to imply that programmers always work on more than one project at a time, in multiple panes, as that's certainly not the case for anyone I know.
As for just using a single window on a project and switching between tabs with the keyboard, as was suggested by the first response: what do you do with all your leftover screen real estate? My TextMate windows are only about 2/5ths the width of my PowerBook screen at ~85 columns (or 1/4th the width of my 1920x1200 desktop display), and when editing source, there's really no need to make the windows much wider. I've bought lots of display area for a reason: I find using it all to be a huge productivity boost, *especially* when programming.
After sending the message last night, though, I did think of an inelegant but very simple short-term solution I think would be at least acceptable (for my uses, anyway):
Adding the ability to open multiple copies of the same project window.
Speaking for myself, if I could clone the project window (or just manually "open" the tmproj file more than once), it still would be somewhat of a pain to position and resize each "pane" of text initially, but once it was done, I could achieve all the advantages of statically arranged panes, quickly switching between documents with cmd-T on each window.
Of course, if the code can support multiple views on the same data (projects and files), I have a hard time imagining this being much easier than just adding panes in the first place. But if it is easier for some reason, I'm all for it, both as a stop-gap solution to the need for multiple views on our work, and for the added UI generality for applications like using multiple monitors of differing pixel dimensions (where you couldn't effectively span a single window).
And thanks to everyone for taking a rather insane post and reponding quickly and politely, with a real discussion. I should have assumed this would be such a nice community. -jrk