I had this situation restoring a database to an SQL Server 2005 Standard Edition instance using Symantec Backup Exec 11d. After the restore job completed the database remained in a "Restoring" state. I had no disk space issues-- the database simply didn't come out of the "Restoring" state.
I ran the following query against the SQL Server instance and found that the database immediately became usable:
RESTORE DATABASE <database name> WITH RECOVERY