Disk utility hanging
How big is your hard-drive? It's entirely possible that it would be normal for zeroing out the sectors to take that long. I'm only extrapolating here, but I've had secure-deletions of the trash take an hour or more for a few gigabytes, so if the hard-drive is large, I could see it taking quite some time.

The other thought is that there is some problem with the drive that prevents the format from completing. You might want to check the drive for errors and/or bad sectors. It is possible that you would have eventually encountered the error on the hard drive yourself (potentially causing instability, data loss, etc.).

Finally, if you look at Activity Monitor, does it look like the disk utility is doing anything? Check your disk I/O statistics there as well and see if anything is still going on with the disk in question.