update 2: I also posted this on the forums and the team finally found what was causing this. They explain:

There have been several reports where intellisense has completely stopped working for all projects after installing a version of the Windows SDK or MSDN. We have been able to track down the source of this problem. This seems to only affect installs of the SDK/MSDN post the installation of Visual Studio. One registry value has been incorrectly reset after these installs causing this failure. This issue has been handed off to setup team for a future fix.

In the meantime, if you encounter this issue, it can be fixed using regedit. First, determine if you are seeing this same issue by opening regedit and looking at the key:

HKEY_CLASSES_ROOT\CLSID\{73B7DC00-F498-4ABD-AB79-D07AFD52F395}\InProcServer32

If (Default) is empty you are hitting this issue. To correct the problem, change the value of (Default) to point to the location of TextMgrP.dll on your system (C:\Program Files\Common Files\Microsoft Shared\MSEnv\TextMgrP.dll in my case with a C: OS drive and accepting all the defaults). Restart Visual Studio and intellisense should be working again. Thanks to everyone who submitted reports of this issue and gave us the additional details needed to track it down quickly.

Update: If you are running a 64-bit version of Windows, you will need to make sure you are running the correct regedit version (%systemroot%\syswow64\regedit - see http://support.microsoft.com/kb/305097) and you will need to locate the correct path to TextMgrP.dll (such as C:\Program Files (x86)\Common Files\Microsoft Shared\MSEnv\TextMgrP.dll)

Update: a repair of visual studio eventually fixed this. Lesson learned: a repair does not damage already installed hotfixes and addins, so you do not have to fear losing anything.

I lost my xaml intellisense and I really miss it. I installed SDK 6.1, but I do not know if that was the cause. I was using Resharper, so I didn't notice. However, when I uninstalled resharper, intellisense did not appear again! It could have been the sdk. It could have been resharper. Or something else all together. I just don't know.

Lot's of googling did not really help (2008 is different than 2005), but did point to the importance of two files in your xml/schemas folder:

- XamlPresentation2006.xsd
- xaml2006.xsd

However, VS 2008 does not work with xsd anymore to supply intellisense. I was under the impression that a Xaml-parser service was build. I do not have those files and on another computer (where intellisense works fine), I did not find them either.

I copied them from a vs2005 install, and opened a xaml file in the xml editor. No schemas were defined (nor are they on the healthy computer), but when I pointed to the just copied files, intellisense does work partially again. It does not see usercontrols and stuff. This is not the way it is supposed to work in 2008!

Funny thing though: I now have this intellisense in the xmleditor, but not in sourcecode editor or the designer. In the healthy install, it's the other way around.

Let me know if you have a solution!