In our development environment we have long been using a particular backup and restore script for each of our products through various SQL Server versions and different environm
We had the same problem going from 2005 to 2008. The problem was that we were trying to use the same backup file in 2008 that we used in 2005 (appending backups together into 1 file).
We changed the script to backed up to a different file and the problem was resolved. I would imagine that moving/deleting the old file would have the same affect