I am new to Workflow and my notes on this session are a little light – but the big news is that WF 4.0 required a major rewrite of the underlying infrastructure such that old WF activities etc… aren’t compatible with the new runtime. You can use an Interop Activity wrapper to enable running older activities so they enabled backwards compatibility in a different way, but interesting to hear about such a huge rewrite of WF so early on in its lifecycle (it is still somewhat of a baby being less than two year since RTM of the first release). I am not sure if you could do this in the past, but with WF 4.0 if you have a WF persisted and do an upgrade of the workflow it will still be able to run the old version of the workflow. I may have butchered the description of what he said, but the demo he showed was persisting (or in other words suspending) a workflow, changing the activity in the workflow, and then resuming it and it was able to run without problems.
That is about it for this session – but it should be said that with 2010 and WF 4.0 and WCF 4.0 (which by the way everyone pronounces Dub F and Dub C F) MS is working hard on making WF and WCF work together better including enhancing WAS (Windows Application Server which enhances IIS infrastructure to be a WCF host for more than just HTTP) to be a host for both WCF and WF.