Great hearing about the ASP.NET 4.0 support! When I did some initial test I just added the controlRenderingCompatibilityVersion and clientIDMode to the pages section of the UI and Util locations and this seemed to be working fine. This as I want to take advantage of these improvements in my templates.
Have anyone found any issues with doing this? Property Control? XForms? Would be especially keen to hear from the EPiServer Team that tested it!