Retrieving a list of DBs that are NOT participating in mirroring

I have a stored procedure that I use on all my servers that queries the master table and retrieves all the data I need regarding each DB siting on the server. I can then decide which files need shrinking, which DB needs more space, etc. I was recently tasked with mirroring. My sp started to fail because a database on the mirrored server acting as a mirror is not available. I changed my query to only grab the IDs of DBs NOT participating in mirroring an viola! My sp worked again. Here's the query:

SELECT D.database_id
FROM sys.database_mirroring M
INNER JOIN sys.databases D
ON M.database_id = D.database_id
WHERE M.mirroring_role_desc IS NULL

I wish I was smart enough to have come up w/ this on my own, but such is not the case. Here's the "credit where credit is due": http://social.msdn.microsoft.com/Forums/en/sqldatabasemirroring/thread/ded375ce-1f09-4070-b588-3d3cb91ed41a