If the dial on top is set to ANYTHING OTHER than M you get full auto exposure ONLY in video mode. No Av, Tv, P or other modes you are used to in still modes.
Now, here's how I set up with both the 7D and T2i:
1. Insure I'm in manual mode.
2. Select shutter speed, usually 1/60th or multiple (In PAL regions that should be 1/50th or multiple) so as not to have a flicker or "banding" problem if artificial light sources are in the scene.
3. Select aperture for desired depth of field characteristic (large for shallow zone of acceptable focus, small for a deep zone of sharpness, or something in between). In bright daylight you may need to have ND filters handy to cut the light if using larger apertures.
4. With ISO set to AUTO press the shutter release lightly to make the ISO value the camera wants to use read out on the LCD (if you press enough to take a still picture you can look at that with it's histogram if you know how to interpret them) and manually set that ISO value (using the ISO button on top of the camera).
Manually setting the ISO insures that it won't shift on you as you pan or as lighting or reflectivity of your subject changes. I want this constant and not changing on me.
At this point you can also try shifting the ISO up or down to change the exposure "look" on the LCD for whatever visual effect you want. CAUTION: To do this and be able to evaluate what you are seeing, you ABSOLUTELY MUST use some kind of viewfinder "loupe" to be able to "see" what is on the LCD with any accuracy. You cannot make good judgements if the image you are seeing is subject to extraneous light, a loupe blocks out all that and lets you see the LCD the same way each and every time.
I recommend the Hoodman Cinema Pro Kit for this (http://www.hoodmanusa.com/products.asp?dept=1066). $209 might seem a bit pricey but it is the best workable solution currently. I find there is minimal adjustment switching between the 7D and T2i.