Until 10 minutes ago, I was running Parallels Desktop Build 5584. I use Parallels so infrequently that problems with it tend to go unnoticed, and unmentioned here. I had, however, been noticing an annoying tendency for 5584 to hog the Mac's user interface. Let me be quite specific: it wasn't (as far as I could tell) a CPU issue—Activity Monitor reported quite modest CPU usage throughout. It seemed to be a pure UI issue. For example, on starting up a Windows XP virtual machine, two things were immediately noticeable: firstly, the five second Windows startup sound played staccato over about 10–20 seconds; and secondly, Command-Tab switching was on about a 10 second delay. That is, I could press and hold Command-Tab (in OS X, not Windows), and expect the application selection overlay to appear about 10 seconds later. Once the VM was up and running, things tended to settle down. But, again, it didn't seem to be a CPU issue—I have more than a few cores to spare on this Mac Pro most of the time.
Anyway, apparently I missed Build 5600, but I found 5608. The "Check for Updates..." feature in the Help menu only seems to work manually. I have it set to check every seven days, but Parallels Desktop never told me there were updates until I checked manually. I tried upgrading within the application, but it failed. Twice. It certainly seemed to be downloading the 88M disk image (twice), but it left a faulty image (weighing in at a mere 16K) on the desktop. Twice. So I downloaded the disk image of the same name from their website. There's no indication on the website (nor within the application) that this is the way to upgrade an existing install (and maybe it's not), but it seems to be working so far. Startup was smooth, the UI was not hogged. I like Parallels Desktop, but they make it a hard app to love.