If you plan to start writing workflows using Workflow Foundation 3.5 (or 3.0 for that matter) definitely take a look at the following links. There are quite a few architectural changes in Workflow Foundation 4.0 so beware:
- Windows Workflow Changes direction
- Windows Workflow Changes Direction: Write New Workflows with .NET 4.0 in Mind
- WF 4.0 – the new runtime model
- The road to 4 – WF changes between Beta 1 and CTP
- WF 4.0 migration guidance - These documents help .NET developers better understand and evaluate Windows Workflow Foundation (WF) in the Microsoft .NET Framework 4, and the impact of WF 4 on existing WF workflows and code.