Thanks, to all! But especially Dave's whose adventures with camera and lens parts are the stuff of legends. So far I have heard two people who believe cleaning the mirror does not involve magic, simply a blower, a padded paddle, and the correct solvent. In my mind the trickiest part will be supporting the mirror, and being very gentle, as it looks very thin.
I'm still just assuming that the sensor cleaner is what could be used on the mirror because it evaporates so quickly, and presumably is very pure.
My experience cleaning the sensor on one of my Canons was anti-climatic. It actually has a filter over the sensor which is becoming less universal the higher the sensor resolution is. And the results were dramatic! You could actually see artifacts in images that were taken at high apertures before, and nothing after. Ah, it appears I have some Eclipse sensor cleaning fluid on hand. I won't be proceeding for a couple of days at least, because, err, I still need my camera to work for a few days or more. But when the time comes I will probably go for it, unless the smudge magically disappears.
Interestingly the Canon 5D Mark II seems to focus the best in live view. But I think that is true of all cameras where your subject is not moving much. Not necessarily the fastest, but the more accurate focus. But I digress. I only mention it because the AF speed is not as fast as expected...in difficult lighting situations...I don't expect miracles...
Brightcolors: It's always interesting reading your comments for technical details, like about the mirror. Maybe it would be accuracy effected, and not speed, because diffraction could be occurring when hitting a translucent substance. If it were bad enough, maybe it would block some of the light going through causing low light performance? All guesswork, but it's always nice when a problem matches up with what you believe to be the cause. Anyway, I will hopefully learn a bit more during this planning phase!