The January CTP of WinFX has now been posted, and there's a possible installation snag you should know about.
As usual, it's a good idea to uninstall previous CTPs in reverse order from how they were installed: Uninstall the Visual Studio extensions first, then the WinFX SDK, and then the WinFX Runtime Components. Then you'll be ready to install the new RC, SDK, and VS extensions in that order.
I thought I had this process down pat, but when running through the steps this morning, the SDK installation balked. I nearly freaked because reformatting a partition was not something I felt like doing today. Fortunately before getting too hysterical, I read the message very carefully. (It sometimes helps! Really!) The SDK component that had not been uninstalled was specifically the "Microsoft Command Prompt."
Sure enough, "Microsoft Command Prompt" was listed in Start Programs and also (fortunately) in Control Panel Add or Remove Programs. Removing that allowed the SDK installation to proceed.