The hard drive knows (or will know) that a sector is bad when you try to write to it, so it will remap that sector to an area reserved for that sort of remapping. When you "cat /dev/null", you're just writing a bunch of zeros to every bit on the drive, so that when it gets to that sector it will remap it. The reason it doesn't do it until you write there is in case the data on the sector is still recoverable.

Hmm. Mark, is there any reason you couldn't just write zeros to that one sector? I mean, it would still corrupt the filesystem, but it would take less time to do it. Or maybe there's not a good way to find where the offset to that sector is.