Came across one of those situations the other day where it takes someone looking over your shoulder to point out the bleeding obvious. An extremely frustrated developer had used the fixMsg SDK sample as a base for a new pipeline component. He got it compiling nicely but each time he attempted to add the component to the toolbox he received an error like "There are no components in <assembly name> that can be placed on the toolbox".
After much frustration he called me over and showed me what he was doing. He would right mouse on the toolbox within Visual Studio and select "Choose Items". Through habit I guess he would then browse for the component using the .NET components tab. There is the mistake. Select the "BizTalk Pipeline Components" tab. If your assembly is in C:\Program Files\Microsoft BizTalk Server 2006\Pipeline Components you should see the component in the list. Simply check the check box and hit ok. Too easy!