Spooky. Bob is talking about UDTs in SQL Server Management Console (SSMS - I've hijacked his acronym). I've had the very same problem, and couldn't work out why the SSMS couldn't see the UDT, but that using ToString() explicitly worked. I had to mail the PM for UDTs to get it answered. It hit me doubly as I also have a User Defined Aggregate for explicit aggregation of the UDT, and that didn't work in SSMS either. Same problem.
During this early testing phase when we're doing lots of build/deploy/test (call it iterative development, it sounds better) this is a royal pain. If you want to keep deploying to SQL Server you probably don't want the assembly in the gac. I suppose the answer is to spend more time up front designing and getting your code right, but that's not always the best way to learn. Well, not for me anyway.