Photographer Michael Reichmann made some 13"x19" prints from the 14.7 megapixel Canon G10 and also from the 39 megapixel Hasselblad H2. Customers visiting his studio couldn't tell them apart as there was almost a 50 / 50 split in opinions. Read all about it on this CNET blog.
For a more in-depth look, the Luminous Landscape article makes very interesting reading.
Having downloaded both files and compared them side by side on a 24" screen, I must say that looking at the two images, its practically impossible to tell them apart! At 100% magnification, things are a little different; the Hasselblad was used on a tripod and the G10 was hand held at 1/8 of a second, so there is tiny movement with the G10 (only apparent at 100%) - but its still a very close call.
The colour, sharpness, tonality and the crispness of the lens on the Canon are astonishing! Who would have thought that a pocket camera could stand up to the best medium format system available?!