The latest high-end laptops are nothing to sneeze at... My MBP has a 2.5GHz Intel Core 2 Duo (dual core) CPU w/4GBs RAM. A couple years ago you might've laughed at the notion of a multi-core laptop or even one that can accept 4GB of RAM. It runs Photoshop and even other OSes in virtualization plenty quick. IMO, they're finally reaching the point where they truly can replace desktops for all but the most hardcore uses.
But it's true, if you've got hundreds of photos to batch process... A quad-core desktop is hard to beat. The other thing that really helps desktops are the hard drives. Laptop drives have come a long way and can reach 7,200rpm - but won't be a match for 10k or 15k drives (not that your average desktop has these however) especially in a striped RAID set.