I'd say it really depends on your shooting technique and your lenses. I just made a 42 inch wide print from a 1DsII file shot with a Nikkor 18mm 3.5 and even looking at it through my reading glasses (finally showing my age) it's looks unbelievably good - sharp, crisp, no signs if pixelation or rough gradations. It actually looks closer to 4X5 than anything else. Your post processing technique has a lot to do with how well your images will print. Raw file processing, interpolation an sharpening are key to getting the most out of your MKII.
This is the type of situation where being totally anal about tripods, mirror lockup, optimum aperture, accurate focus and specific lenses make a very noticeable difference.
I just made a bunch of 32 x 48 inch prints for a friend scanned on his Howtek at 4000 dpi from Kodachrome originals, and they looked great. They were not nearly as sharp as the 1DsMKII file I just printed, but still looked great in that wonderful organic film sort of way.
I would say that if you're careful and have the right image, 40 X 60 inch prints from the MKII will still look great.