I discovered that

System.Windows.Media.RenderCapability.Tierreported a wrong tier (zero) after I upgraded my Windows XP dev box’s .Net Framework 3.0 installation to the RC1 release. My card used to be a tier two and I had just upgraded the drivers, too. I contacted Microsoft about it and they said this is a known issue with the RC1 release and that it will be fixed in the upcoming RTM release. The problem is only with this particular API and only happens on Windows XP. All the actual rendering is still done correctly based on the capabilities of the hardware. So the only problem is if your WPF application is built to behave differently based on the value of this API call.There is a workaround that has to do with changing the value type of a certain registry entry (the video memory size) from Binary to DWORD. I’ll point to more details once I get a URL from Microsoft to point to.

Update: Here’s the link to the official release notes for this problem:

http://msdn.microsoft.com/windowsvista/support/relnotes/netfxRc1/default.aspx#topic6 

Also, I need to correct my statement about this only happening on XP. That’s not entirely correct. It happens with XPDM drivers, which you can have on Vista as well. It’s just more likely that you have XPDM drivers on XP.