I agree with the quality observations above. When pixel peeping you will see differences in quality, but once on paper the differences even up nicely until you get to extra large prints. On the screen, its not even an issue.
IMO, the risk one takes when shooting on the 4/3 systems is when they happen upon an image that they later want huge prints done. In those cases the content usually supersedes the quality, and there are ways to up-res to improve the quality for large prints.
The other way to mitigate that risk is to drag the DSLR around everywhere just in case. But I feel the 4/3 system is perfectly suited for those that are unwilling to do so, and looking for something better than the P&S offerings for every day carrying.
I would also be curious to figure out what the size relationship is for a full frame sensor compared to a huge billboard, and what an equivalent size print/billboard would be for the 4/3 system. I would bet is a very large print, and would look just as good as the proper viewing distance. I recently shot an advertising job where the images were used on billboards, the full sides of buses, etc, and I was worried that the 1DsMkii wasn't up to the task. Once seeing it in person, that was quickly laid to rest. I feel the 4/3 could easily do a 1/4 of that billboard with similar results, and that would exceed most print sizes I would be likely to use.
Here is a shot of how big the billboard is, and the quality was excellent. You can see the awnings and doorway of the building in the lower right corner as a size comparison. That's one monster size print! I think we tend to forget how big we can print images and have them look nice at the proper viewing distance. Any big print is going to show artifacts when viewed up close, but those disappear when you step back and take in the whole image at a reasonable distance. I think I should make some 40x60 prints off the u4/3 as a test!