I do not know the complete cause for the broken intellisense in my Silverlight XAML editor. It could be installation of the windows SDK or the Latest MSDN but to fix it is relatively simple ..
Locate the TextMgrP COM library and re-register it. I located it at
C:\Program Files\Common Files\Microsoft Shared\MSEnv\TextMgrP.dll
Regsvr32 “C:\Program Files\Common Files\Microsoft Shared\MSEnv\TextMgrP.dll”
and intellisense should be back and operational