Guess none of you noticed your system resources are being used quite heavily when you fire up DU.
Go fire up Activity monitor and set it to sort by CPU usage, then hit Disk Utility and do a Repair - you will see a job started by _installdb that eats up CPU.
I believe it's to reindex the installed applications and everything before actually attempting repair. But yes, it's taking _too_ long, so I would classify this as a bug.
I suppose that, thanks to changes in the underlying architecture of Leopard with regards to hard links for folders, extra checks like this are extremely necessary (unless you want to ruin your Time Machine backups with a repair that is not aware of the architecture changes in the file system)