This might be something that all the experts here know about already, but I was stumped by it initially, and was happy to figure out a solution. I had an NTSF-formatted Windows XP hard drive in a PC box, and I was too lazy to plug in a keyboard, mouse, and monitor. Instead, I pulled the hard drive out of the PC and plunked it into a USB2 enclosure, which I then plugged in to my Mac. I could read the disc, but when I went to Disc Utility to wipe it and reformat it, the option was greyed out.
After some futile searching, I was ready to boot up off of an OS X install disc (as one hint had suggested), but then figured I'd try one more thing -- I ejected the volume, and then, voila, I could format it as an unmounted volume. Easy!
I can now donate the PC to a youth artist group, knowing that the person who gave me the PC in the first place won't have all their data out there in the mean old world. I did a 'write 8 times format' in order to clear the drive. I point this out just in case anyone else decides to do this and accidentally puts a chunk of their personal data out into the world.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050419060913995