If you're just trying to limit the number of databases that a user sees to those that they actually have access to, you might want to consider installing this updated version of the sp_MSdbuseraccess stored procedure :

http://support.microsoft.com/kb/889696/en-us

Essentially, when EM populates the list of databases it goes through sysdatabases and pulls out each database name it finds. With the modified version it also adds a check to see if the user has access to each database, and only displays those which return as true.

Note that this only works through Enterprise Manager, a user connecting from 2000 Query Manager or 2005 Management Studio will still see all the db's since they use different methods to get the database list, so doesn't ensure users cannot see which other databases are in place, but does at least reduce the number of non-relevant databases they are shown.