I'm posting this because a few people seem to be confused about how to update their .NET based ActionStudio actions for FB 6. Even though there is a new API for writing .NET based custom actions, you don't need to use this - you can still use the old API.
I'd still recommend the new API if you're writing new actions.
Here's an excerpt that explains the differences:
The new interop namespace is VSoft.CustomActionApi.Interop, instead of the old Interop.FB50CustomActionInterfaces. Also, the interface names are now ICustomAction instead of IFBCustomAction (and similar.) However, the interface definitions should be identical, apart from minor changes to support new FB 6 features. The namespace/interface names for Interop are now "future proof" as well, so they no longer change with each new major version - you should be able to just recompile against the new interop assembly.
The new API VSoft.CustomActionAPI.dll is designed to make it quicker and easier to write new actions, but we're not requiring it for updating legacy code.