We've seen this on 3 machines here.. Outlook 2k pst files appear to have no information in them anymore. The size on disk will be 40MB,100MB, etc. Within outlook when you check the folder size, the only folder that will show under the pst file will be deleted items and the file size (w/in outlook) will read as 0 bytes. If you open explorer and look at the file you will see it start to shrink. A few kb every couple seconds. The only way to stop the shrinking is to close outlook.
We've explored the idea of a virus - not likely, they are all running up to date sig files. At this point, I have absolutely no idea.
I almost forgot, we've of course run scanpst.exe and the machines are not fragmented either. They run diskeeper every night.