Actually, to be clear, I've been seeing this problems from about build 917 or so.
Essentially, intermittently, Find all Usages -- even on a usage of a method -- will display the No Usages Found! error. Restarting Visual Studio generally fixes it again... until it breaks later. Needless to say, this is really annoying, and will diminish my enthusiasm for the 4.0.1 release considerably...
Anyone else seeing this? More importantly, any idea what assistance I could provide to help narrow down the problem? (For instance, are there any relevant logs that I could forward?)