Phantom Files, Filesystem Full

[Ryan Winograd]

I was editing a conf file in /etc and, when i tried to write the file vi 
told me it couldn't save because the filesystem is full! I ran df and, 
sure enough, the / fs has 245M of 258M used:

Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ad0s1a    248M    245M    -17M   108%    /

This was certaintly a surprise, because all my data is located on other 
partitions, so i ran du -h -d1 / to find the culprit, but i still 
couldn't find what is using up all my / disk space! What can i do to 
find out where all my disk space has gone?!?!

Thanks for all help
ryan