I wouldn't bet that "better" lenses have less vignetting. The 24-105/4L IS USM for example shows significant vignetting at 24mm. Vignetting is a fact of life for fast lenses, but luckily you can correct it pretty easily if you shoot RAW and use DPP. The 24/1.4L also shows pretty strong vignetting at f1.4 abd at f2.8 it's similar to the 24/2.8. The 50/1.2L is really no better than the 50/1.4L at apertures of f1.4 and slower.
Edge and corner image quality are something to consider, especially if using fasy wideangle lenses. However remember that with most fast lenses used for things like portrait work (50/1.2/1.4/1.8 or 85/1.2/1.4/1.8 for example) the corners of the image will probably not be in focus due to DOF limitations.
No, I think the 5D MKII and 50D both have "peripheral illumination correction" as an option when using a supported Canon lens (but not 3rd party lenses). It's applied to JPEGs but doesn't change the RAW file.
That is why I said "some digicams", I know Canon does not do this, but I recall a story that Adobe was "persuaded" to incorporate this in their software to cover up the hardware defect.
If you're refering to P&S digicams, then I think you're right. I don't think any of the Canon Powershots can correct for vignetting. I'm not sure why since it would be easy on a P&S with a fixed lens since the lens could be fully characterized with much less data than Canon needs in order to do it with a few dozen lenses for an SLR. It's all software and the routines must have been written for DLSRs so you'd think the firmware wouldn't cost much to develop.
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