velocipedio

I'd hardly call cyclocross bikes a re-evolution of the MTB. The sport of cyclocross has been around since the mid-1940s [after the war, although I understand that there were some organized races in liberated Belgium as early as December, 1944...]. If anyhting, mountain biking is a re-evolution of cyclocross.

Having said that, there are some major differences between the two sports at a competitive level:
[list=1][*]Cyclocross courses are short loops of 2.5-3.5 km, while XC mountain bike courses tend to be considerably longer.[*]Cyclocross races run for an hour, while XC mountain bike races run for a few hours.[*]Mountain bike racers are not allowed to accept technical assistance from anyone during the race; cyclocrossers can have up to three spare bikes in the pit.[*]Cyclocross courses are designed with barriers and obstacles that force the rider to dismount and run with the bike, XC mountain bike courses are not.[/list=1]
Finally, cyclocross is a winter sport -- okay, fall and winter -- mountain biking is mostly a summer sport. I think one of the things that really defines cyclocross is the fact that it's ideally done in really bad weather conditions -- rain, snow, mud -- that really test the racers.

It's all about the boue...

I really don't see 'cross bikes being adapted from MTBs in any great numbers, though you can ride MTBs in most 'cross races. Most 'crossers seem to like the drop handlebars and there's really very little advantage to having suspension on a 'cross bike... and I see suspension as the defining feature of the MTB. Remember, you have to carry these bikes; suspension is just added weight.