I'm very familiar with flash photography from film days, and I use it with my compacts (G10 etc). I just don't (yet) have a low voltage flash suitable for mounting on my 5D. I bought the camera for its available light potential and that's how I mostly use it. However, as I'm shooting an open-air wedding shortly I now have no option, and I'm getting flashes/triggers/diffusers/reflectors etc to use them off-camera.
But as I said, the T2i like most crop-sensor cameras has an on-board flash, and I wouldn't see the purchase of an off-board flash as high priority compared with getting lenses. I didn't say one should never be bought, just that it can wait a while.
On a completely different subject. You have written elsewhere in considerable praise of the Canon SD4000. I looked it up along with the SD4500, expecting the SD4500 to be a bit better across the board. Instead I see they're really different cameras, and the widest the SD4500 lens goes is 36mm (35mm equivalent) whereas the SD4000 goes to 28mm (on the same basis). And the SD4000 lens is at its widest f2.0 (the same as the S90/95) whereas the SD4500 only goes to f3.4. In almost every respect the SD4000 seems the better camera, yet it's cheaper.
What do you know of the real-life low light performance of these cameras? I wouldn't expect much of the SD4500, but how does the SD4000 compare with the S95?