Just installed the Visual Studio RC and have come across the following issues.
System.Runtime.Persistance has been replaced by
System.Runtime.DurableInstancing
All the Workflow Persistance Schema and Logic has changed and there is no Migration path. You can find the latest scripts here:
C:\Windows\Microsoft.NET\Framework\v4.0.30128\SQL\en\
These were the only issues I had when upgrading a 30 project solution.
Really enjoying the increased speed combined with ReSharper 5.0