I've complained here and here that I could not run Tools | Update Center. Trying to run Update Center would result in an 'unexpected exception'. The problem appears to be one or more corrupt settings in the .netbeans/dev cache area. I don't know where the specific problem lies, but I was able to test this theory by simply renaming .netbeans/dev to .netbeans/dev.bad and restarting the 4.2 Q build. When I forced NetBeans 4.2 to rebuilt its cache area, then the Update Center started working again.
I have yet to determine exactly what the problem is, but I did manage to carefully move many of my older settings from the 'bad' dev cache to the newer, so I could at least continue with my projects and my database settings and servers under Runtime. The Update Development Center is now open to me again. As I posted earlier the problem showed up with a daily build, specifically the one from July 24th. Getting rid of the build didn't get rid of the problem. If you run into something like this in the future then as a sanity check rename or move the existing NetBeans cache and re-run Netbeans to see if the problem goes away. This is, interestingly enough, very similar to a problem I had with developer builds of Eclipse 3.1.
Thanks to Jiri Rechtacek for pointing out the solution.