You may not have noticed yet but the December CTP of Windows Vista is now available for download to MSDN Subscribers. This is important as it is the first version available to MSDN subscribers that supports the RTM version of Visual Studio 2005 and the compatible WinFX bits (including the Windows Communication, Workflow and Presentation Foundation bits).
Note that after you install Vista you will need to enable the WinFX runtime this is disabled by default in this version.
You can do this by going to the Vista Software Explorer, checking the box next to the WinFX runtime and click ok.
Do this before you install VS2005 the Windows SDK and the Visual Studio Code Name “Orcas” CTP WinFX Development Tools.
This CTP contains the December CTP of the WinFX runtime... and yes these bits are available for download for Windows XP. The good news is that the Dec CTP WinFX runtime is compatible with the Nov CTP WinFX runtime... these are simply different dot builds of the same release containing additional bug fixes as required to maintain compatibility with Windows Vista... so if you have already got a full environment set up running the Nov CTP on Win XP you may not need to upgrade to the Dec CTP.
*update* Robert McLaws reports that it may be a couple of days before it actually appears on MSDN. I just logged in and I am still seeing build 5231. A new page has been released to the Windows Vista Developer Center http://msdn.microsoft.com/windowsvista/getthebeta/default.aspx.