CRM 2011: Trouble registering plugin

I upgraded my CRM 4 plugins to CRM 2011 and when I try to register using the plugin registration tool I get this error:

Plug-in assembly does not contain the required types or assembly content cannot be updated

My namespace, class, and dll name all remained the same. I removed all references to CRM 4 dll's and tried using an on disk registration with the same results. Any idea what I'm missing?