HI tiagobt,
[...] However, as soon as I restart Ubuntu, the resolution is set to the highest one available (3360x2100). Anyone else having this problem?
I just tried and did observe this as well. Ubuntu 13.04, I set the resolution to 1920x1200, Suspend the VM, resume the VM, it changes the resolution to 2880x1800. (mine maxes out there, which is what it should do). I'll let the engineers know.
Using this resolution, everything looks small, but the image is not very sharp. It looks as if the operating system were trying to simulate a high resolution on a low resolution display (which is not the case). Using the resolution 1680x1050, the size of the text and images is fine, but things are not as sharp as they could be.
It actually is the opposite case. You're rendering a low-pixel-density image on a high-pixel-density display. When it's tiny, it is rendering pixels at a 1:1 (or close to it) ratio. There are actually 2880x1800 physical pixels in the Mac's display. Linux doesn't have the ability to be more 'dense' like how Apple has tweaked OS X for this display. Same reason that Windows looks chunky at 'normal' resolution, and tiny at 1:1.
I've also tried to use the highest resolution available (3360x2100) and configure Ubuntu in a way that the the size of text, images and apps is not too small. However, I haven't managed to configure everything properly. Most apps (like Firefox) display things smaller than they should. Any idea how to configure Ubuntu properly in a high DPI display?
Ubuntu isn't designed for a High-DPI display, so anything you do to shoehorn it in is going to be met with compromises. If they had a way of specifying the rendering density of the GUI, you could set the resolution to 2880x1800 and Ubuntu would make things sized/proportioned like 1440x900 but be super crisp.