As it does the calculation, it makes sure all the files are accessible and locks them all down so they can't be moved while its in the middle of copying. That way, during the copy, there's no surprises like in XP where XP gets 80% of the way done, the aborts because of some read-only file, leaving you with a partially copied directory where you're not really sure just which files got copied and which didn't - particularly annoying in XP when you were trying to do a "move" and not a "copy".
I like the idea in Vista, but I *hate* the time it takes. I'd rather just take my chances and have it work the same as XP. Or at least let it be enabled/disabled. (Heck, maybe it can be; there's probably lots of little back door registry tweaks in Vista that we don't know about yet.)