Scaling 1920x1080 down to 960x540 still gives a higher pixel density than on the Pandora, so I think that would be a good default (coincidentally, that's the resolution of the Vita). It would make sense to define a few default resolution settings for X, including fullHD, 960x540, and maybe 800x480, 1024x600, 1280x720 and 1366x768. You could toggle between them with CTRL-ALT- +/- or some other shortcut. For some applications you may want to use a high resolution (e.g. a browser, which scales everything nicely anyway), for others a lower resolution would be more appropriate (e.g. a Pandora application that was intended for 800x480). Also if you're going to connect to a monitor or beamer, the ability to switch resolution will be important, so you can make the Pyra screen match the monitor screen.