I was working with an old custom workflow activity and some documentation I had for installing it. In an old DEV environment (all in 1 box), all I had to do is deploy the DLL in the GAC. However, I noticed in a tiered environment where FIM components are installed on different boxes, it seems it must be GAC'd on those boxes as well.
So is this statement true? -> The custom workflow assembly needs to be deployed to all servers that host the FIM web services and Portal components. The Portal needs it for the WebUI controls, and the FIM Web Service needs it for running the actual activity?