Here is an article that explains why, and uses the same solution Mike already posted.

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q274188




>We've got our new SQL server up and running and I'm trying to copy our
>database from the old server to the new server.
>
>I detached the database from the old server, copied the data and log files and attached it to the new one. I can see it and navigate it just fine through enterprise manager but I cannot log into it through sa. It says that owner 'dbo' is not a user for this database. However, when I try to add the owner to the database it says it already exists.
>
>Are there any steps I'm missing for moving a database from one server to another?
>
>Jake