I was at a customer the other day, and something interesting came up. The server was SQL Server 2005 - an in place upgrade from 2000 - and we were troubleshooting a performance problem. Of course, I wanted to select from some DMVs to see what was ailing the SQL Server; however, an error returned stating it could not find an object. I was sure I knew the name of it, so I was a little confused. We started loooking at the database options, and found that master was still in 8.0 compatibility mode. All the other databases were 9.0 compatibility, including MSDB. It was strange that master was not converted, as well.
We were able to change the compatibility mode to 9.0 and as you might expect, the functionality we expected, returned. This is definitely something to watch out for in future upgrades, though.
Jon