Ok, using sp_helplogins gets me a lot closer for what I was looking to do. Basically, I've written a front end that needs to check all of the existing dbases on the server to generate a list of what they should 'see.'
Reading through the sp_helplogins stored procedure though, I've noticed that only sysadmins can run this, and the stored procedure creates several temporary tables to store the table before returning the results.
Is there a way to structure this using only the sysdatabases for a dblist, and somehow looping through each database to determine the roles / users who have access?