I'm interrupting my three-parter with a short post regarding something I learned just the other day. I was originally under the impression that in order to deploy a pipeline component, it must be placed in the Pipeline Components folder of the BizTalk installation directory. Apparently, this was the rule in BizTalk 2004, but it has since changed - pipeline components can now be GACed as well.
It's my understanding that the reason that the Pipeline Components directory exists is because developers need a standard place to put their components after they develop them so they can be easily added to the Visual Studio toolbar and subsequently used in new pipelines. It's also handy because you can easily deploy a debug version of a component there and step through the code in the debugger once the pipeline runs by attaching the VS debugger to the BTSNTSvc service corresponding to the host instance running the pipeline.
As Stephen Thomas points out here, make sure you know and understand where the component is going during development, where it's going in deployment, and that those two strategies mesh. He points out a scenario he ran into in which he built a pipeline component and added it to a pipeline before strong-naming it and GACing it. When he tried to deploy, the operation failed because it couldn't find the pipeline component.