I found this comparison of 4 x 5 and PhaseOne P45 39 megapixel back.
http://www.luminous-landscape.com/essays/Cramer.shtml
Given that a Hasselblad H3D-39 on promotional offer, with a 28mm lens and voucher for a second lens runs to just shy of £21,000 excluding VAT
, I can't see many of us taking it up.
Interesting comparison though, and with 5 x 4 having 3.5 times greater area than 6x6cm. This would suggest that a Canon EOS-1DS MkII, at 16.7mp would give 6x6 a run for its money.
Don