[robg adds: When I first tested this, it didn't work for me -- I couldn't open another movie (via drag and drop, double-clicking in the Finder, or using QuickTime's Open dialog box) while the export was going.
Then I tried minimizing the export window to the Dock. Once I did that, then all the usual methods of opening a QuickTime video worked just fine. Using this method, when I started the next export, nothing seemed to happen. But that's because QuickTime uses the already-existing Export window, and just adds the second movie below the first. Since I had the window minimized in the Dock, I couldn't see that.
An anonymous tipster wrote in that "each new export spawns an entirely separate process called QTplayerHelper. Interestingly, this process is given a nice value of 3. Exports can therefore be sped up a bit by re-nicing said process using the Terminal."]

