Today the WF team released a migration kit to wf.codeplex.com
As it says on the CodePlex page, the WF Migration Kit helps users migrate WF3 (System.Workflow) artifacts to WF4 (System.Activities). The high level goal is to migrate workflow definitions and declarative conditions, but not code (such as workflow code-beside methods). Some WF3 workflows will be fully migratable to WF4, while others will be partially migratable and will require manual editing to complete the migration to WF4.
The WF Migration Kit provides an API as well as a command line executable tool, which is a wrapper around the API. Migrators are included for some, but not all, of the WF3 out-of-box activities. The WF Migration Kit offers an extensibility point so that custom activity migrators can be developed by third parties.
For information about migrating WF3 artifacts not addressed by the WF Migration Kit, please see the documents posted on MSDN here.