If you create a maintenance plan (SQL 9.0.3042) to backup databases and select All databases as below;
Then at some point in the future if one of the databases is taken offline the job created by the maintenance plan will fail.
From what I have seen the job will still backup the rest of the databases (which is the only saving grace), you just get a lot of errors in you SQL log like the below;
BACKUP failed to complete the command BACKUP DATABSE SQLServer365. Check the backup application log for detailed messages.
Shortly followed by;
BackupDiskFile::OpenMedia: Backup device 'D:\Backups\SQLServer365\ SQLServer365_backup_201203160500.bak' failed to open. Operating system error 2(error not found).
Yes I know, you should know that the database was taken offline and have amended the maintenance plan accordingly. In my experience though I have found that people create maintenance plans as a quick temporary fix on non-critical servers (which becomes permanent). These servers are often forgotten about until there is a problem.
I avoid creating maintenance plans, yes they are quick and easy but are often everything or nothing. You are much better creating maintenance routines tailored to your environment. I am in the process of creating a maintenance routine for backups, consistency checks and index maintenance to work across all flavours of SQL Server with a whole host of configurable parameters.
I will provide this once complete.