Having shot the D3x for years, seen files from several backs printed well (although nothing both modern and really high end - my experience is looking at prints from 39 mp class backs, NOT the 60s and I've never even seen an 80 mp back), and played with and seen output from the Sony and Canon 20+ mp DSLRs, here's what it looks like in my (admittedly subjective) view.
1.) The D3x is significantly better than any other "35mm" type DSLR. There is extra sharpness and DR in its images that just isn't there in other FF cameras. This may be the AA filter, the (slow) 14-bit implementation, or something else, but I can reliably pick a D3x print from a 5D II or an Alpha every time (assuming the D3x is handled right - ISO 100, 14 bit, tripod, really good glass). Of course, a D3x handled like this is a 1.5 fps camera, just like a fast MF back (it is a little lighter, weatherproof (perhaps indestructible), and has a broader range of lenses, but it is not really any faster than a P40+).
2.) The D3x is roughly equivalent to well-handled medium-format film, shot at 6x9 cm. I've compared prints extensively between the D3x and Hasselblad V-series gear shooting Velvia, and the D3x has a clear and significant edge. 6x9 becomes very close.
3.) The D3x is edged out in ultimate detail, in a very large print, by a 39 mp back (even one from 2 years ago, when I made this comparison and bought the D3x), and is roughly equivalent in other ways. Dynamic range is very similar, unlike other small-format DSLRs, and color rendition is different, but equally high quality. I actually put the D3x midway between other 20+ MP DSLRs and the backs.
4.) If Phase One's claims and Michael's and Mark Dubovoy's testing are true (I have no reason to believe they aren't), the newest generation of backs should very easily beat the D3x in resolution (not an edge-out situation, but an immediately obvious advantage), and should have slight advantages in most other areas. It seems as if the per-pixel quality of the latest backs is at least at D3x levels, and some of them have more than triple the pixels. The 80mp backs should be roughly equivalent to 5x7 inch film (a bit more than triple the area of 6x9 cm).
5.) The ultimate in image quality is still probably 8x10 sheet film, but an IQ180 beats any film short of 8x10!