I recently purchased a 1Ds III and a buddy of mine tested his against his Phase back. (a P30)
The results were shockingly good. The Canon with an L lens compared quite favorably to my friends Zeiss glass. Granted, the file sizes were different, the overall sharpness of the Canon file was very impressive and I doubt that you would see much difference in CMYK magazine reproduction. In large prints, I believe you would, but for everyday commercial or editorial work, Canon seems to have hit the market with a tool that is easy to work with, reliable and affordable for many pro shooters.
I don't' want to stir up the Medium Format is better pot, I'm just posting my observations. I've shot with the 1Ds II since it was released and was not happy with the files. The 1Ds III is a much better camera. You can see the difference in the highlights and shadows. I have also owned and shot with several medium format backs. There is a difference with the Phase and Leaf backs. The Canon 1Ds III is a fantastic system and everything works well together. That is critically important for location shooters.