It has been widely held that the Four Thirds system offers better corner sharpness than does traditional 35mm photography, so as a user of both systems I thought I'd do a simple test.

Systems compared:
1) Canon 5D (35mm sensor) with EF 28mm f/2.8 prime. This is a cheap lens with sharper corners at 28mm than the more expensive Canon 28mm f/1.8 copy I previously owned. Obviously this lens does not represent the ultimate in corner sharpness available for the 5D (that would require some very expensive glass), but it competes well with the much more expensive L zooms at this focal length (confirmed by SLRGear testing). Look here for a comparison of 13 lenses at this focal length tested on a 5D. You can see that the 28/2.8 is no slouch.
2) Olympus E-410 with 14-42mm zoom (kit lens)

In terms of lens and body, I was somewhat unfair to the Four Thirds system here. The 14-42 is outperformed by the higher cost Olympus zooms (also confirmed by SLRGear testing), and the E-410 has less per-pixel sharpness than the Panasonic DMC-L10 or Olympus E-3.

The 5D was at ISO 800 with the 28mm lens stopped down to f/11
The E-410 was at ISO 200 with the kit lens at 14mm and f/5.6
This was done so that the two systems would produce images with the same depth of field, diagonal angle of view, perspective, and apparent exposure given the same shutter speed and print size.

Shutter speed with both systems was 1/250s. Unfortunately I left my tripod at my parents' home, so both systems were shot handheld. I picked the best of four shots in each case. In each case, all four shots came out the same. I can reliably handhold a 28mm lens at 1/250s.

For processing, I converted from proprietary RAW to DNG in Lightroom and then processed in C1 4 beta using default sharpening (same for both cameras) and zero noise reduction.

More points to consider: 1) To compare these images, they should be examined at the same print size. I did not upres the E-410 photo or downres the 5D photo because the method used to do either could potentially introduce bias. Of course you can do this on your own. I printed them for comparison. 2) C1 doesn't have a specific color profile for the E-410. Color differences here are not representative of these camera models IMO. 3) These two lenses do not represent all lenses available for their given systems (duh). Again, this is a cheap prime designed decades ago versus a cheap zoom designed recently.

Click here for the two images with full-res version and EXIFs available.

Here they are as resized by Flickr. You can see the smearing in the extreme corners of the Canon shot even at this size (Click image for the intended viewing size):


In contrast, corner softness is barely apparent on the Olympus image at this size (Click image for the intended viewing size):


Here's a representative corner crop (Click image for the intended viewing size):


This crop shows the greater overall resolution and accutance of the particular Canon system used in this comparison (Click image for the intended viewing size):


My observations: 1) Softness in the extreme corners was clearly present on the Canon image and much more subtle with the Olympus image. 2) The overall detail rendered by the Canon system was greater. 3) Noise levels were comparable.

Again, this is just one comparison. While I found the results interesting, more lenses and bodies from each format would have to be compared to reach significant conclusions.