George, in the original post dbajunior said "I uncheck db_owner, then click OK." Well in my version of SQL Server 2008, the login mapped to dbo is not listed in the users list of the databases except as dbo, you can not map a login that is already in the database as another user as dbo, and the in the user mapping area of the logins properties, trying to remove the check for db_owner generates an exception 'cannot use special principal dbo' so he would never be able to successfully click "OK".
dbajunior; In your original attempt were you looking at the user as listed in the security\users folder of the database, or the user mapping page of the login properties under the server's security\logins folder?
Also just for fun and giggles why don't you run "sp_Changedbowner 'sa' " in that databae. Then try see if your user in listed in the security\users folder of the database. If it is not then it was mapped to the "dbo" user. If you see it there then that was not the issue.