One other user reported a similar problem on Windows 2000 Server [1]. Since that is the only environment where this problem has been reported (for the other user, the same scheduled task worked properly on another machine [not sure which Windows version]), I suspect there is in fact a bug in Scheduled Tasks on W2K Server (it could also be a bug in VBP that only manifests on that platform, but there have been many known bugs in Scheduled Tasks, so I'm leaning more that way). The thread below provides some alternatives, and we'll do some testing on W2K Server to rule out a VBP bug being the cause.
[1]
http://www.kinook.com/Forum/showthread.php?threadid=548