Easiest thing to do is use Niceplayer's resizing from the center, which would preserve the aspect ratio but crop some pixels, or aspect ratio stretching (it's 16:10 for most Apple laptops, if I'm not mistaken). VLC can force a different aspect ratio as well, and is the only option I know of that you won't have to enable individually for each movie you play (though you'd probably want to disable it for 4:3 movies.)
If you want to not have black bars in QuickTime Pro or Frontrow, you have two options. First, you can change the scaled size of the video track to one with the same aspect ratio as your screen. Second, you can add a rectangular mask to crop the video track to aspect ratio you want. Either way, you'd need to save the changed movie as a QuickTime movie, reference if you still want to keep the original file, and open that one in Frontrow. It's better than reencoding, since there's no quality loss, but still not too fun.
The easiest thing would be to just accept the bars you get when playing a 16:9 movie on a 16:10 screen.