Reflector 18.104.22.1682. The first time you open the GAC (File -> Open Global Assembly Cache...) in a session, if you have a lot in your GAC the assembly list appears to just hang with no visual feedback anything is going on.
On subsequent loads it does that "incremental load" thing that I'm used to where the list starts populated and newly discovered assemblies pop in as they're discovered. The "block" only appears on first load.
Note that the UI is still responsive, it's just that you can't tell anything is happening without looking down at the hard drive LED... which isn't present if you're connecting over Terminal Services.