ConfigMgr 2007 primary site server backups were failing at a customer site. "SQL Writer not found" error in smsbkup.log. CM2007 SP2 on one server, SQL 2005 on a shared SQL server.
SQL server hosted lots of databases, including ConfigMgr. That is almost always a bad idea and continued to be so in this case.
After lots of troubleshooting, including looking at output from 'vssadmin list writers' and getting 'sms writer' with state of  Failed, changing the account that vsswriter was using, restarting the services, etc etc. began looking at the other databases on the SQL box and noticed one had a trailing space at the end of the database name. Very hard to discern this from the management studio, easier to detect it by looking at the SQL logs. Initial thought was it was causing SQL maintenance tasks, including the backend processing for CM backup, to fail. Initial thought was correct. Admin removed the trailing space from the offending database, retried the ConfigMgr backup and voila, backup worked. So, trailing spaces in database names are a bad thing. Avoid them.