A # of people on the 10D forum had this problem, and I'm sad to say the issue has followed me to 11D.
My SQL jobs always return the DB was in use and unable to be backed up. Included with this job are sql instances on other systems which work fine, however on this x64 server running sql 2000 they bomb.
The other servers are of course x86, so there's obviously some issue with 10D/11D and backing up SQL 2000 running on 2003 svr x64
Hopefully someone will respond to this post b/c the half dozen of us with this same issue on 10D were never given any responses.