I just posted a new tech article about the FinalBuilder 6 .NET Custom Action API.
If you've ever done any custom action development with .NET, then I strongly suggest you check this out. It's a really nifty API and it makes designing FB 6 actions incredibly simple.
In addition, it is still possible to tweak the API slightly to add functionality before FB 6 goes final - so please let us know if there's anything that you'd like to see added.