Ugh! I've wasted the past hour trying to figure out why I no longer had IntelliSense while hand-editing my App.Config file in Visual Studio 2005.
Among all of the noise from my search results, I found an old newsgroup post stating that the XML Schema file that VS2005 uses for .NET config files is kept at:
C:\Program Files\Microsoft Visual Studio 8\Xml\Schemas\DotnetConfig.xsd
Hmm, I don't have that file. Could it be that this was an old file used by a beta of VS2005, and that it's just not distributed in RTM?
I IM'd Dustin, and sure enough, he had that file on his machine. So, something was not playing nice, and actually DELETED it from my machine. The nerve!
My solution: Download a new one to that directory (there's an improved version here), and wouldn't you know: IntelliSense started working again!