I have an install of SQL 2008 R2 Express with a major issue. The only sysadmin account is SA and that has been locked out. the only other login is BUILTIN\Users with public access. I have tried using the -m switch but when I try to use SQLCMD to login I get an error "Msg 18461, Level 14, State 1, Server SERVER1\SQLEXPRESS, Line 1 Login failed for user DOMAIN\user'. Reason: Server is in single user mode. Only one administrator can connect at this time." and I am the only trying to connect. I have tried this with both a Domain Admin account as well as the local admin account.
Check this post for a few ideas
http://social.msdn.microsoft.com/Forums/en-US/sqlgetstarted/thread/16706dec-209f-42c4-8dc7-4ee4dd5df482/