I have used both m4/3 (E-P1, E-PL1, GH1, GF1) and Canon APS-C (virtually everything up to and including the 50D/500D). First thing I have to say is that I see nothing that comes close to m4/3 yet as a combination of size, quality, and product range. It is as close to a mature system as you will find in a compact system camera, and even after almost two years of using m4/3 I am still amazed the difference in weight and size between equivalent body and lens combinations in the DSLR and m4/3.
While I have preferences for different purposes, I rarely specifically choose one camera over the other for a given application. There is enough overlap between the two that you could really choose one system or the other and be happy. There is a different look between m4/3 and the Canons which I think is partly due to differences in sensor size/focal length and partly due to the sensor hardware and processing software characteristics. It's not always a case of one producing better looking results than the other, just different looking results that may be better or worse for different reasons.
One thing that I do believe is that is unfair to lump all m4/3 cameras into the one basket for image quality. Each one I've used is different in it's file output (and I'm talking raw, not jpeg) Subtle in some cases, but still noticeable. For noise I rate from best to worst the GH1/E-PL1, E-P1, and the GF1. Colour output (subjective, of course): E-PL1/E-P1, GH1 very close, then the GF1. Dynamic Range: GH1 clearly in front, E-PL1/E-P1, then the GF1. Resolution: E-PL1, GH1, GF1 (prior to applying noise reduction which seems to affect this camera the most), then the E-P1.
I think that the Canon file quality comes out in front of all of them, but the GH1 comes the closest to matching it, losing out mostly in having more noise for any given ISO. I haven't tried the newer sensors in the GH2/G3/GX1 so I don't know where they sit relative to the GH1. I think they are better for higher ISOs but the numbers don't suggest that they are any better at lower ISOs.
If I had to choose one camera it would probably have to be the GH1 which is easily the best compromise I have experienced for everything from image quality, to size, handling, responsiveness, and features.