One of the first things I'll do is hook it to a 3840x2400 display via HDMI->DVI adapter. Ubuntu drove it out-of-the-box at full-resolution from a single-link DVI at 12Hz refresh rate.
Any bets on if it'll actually work for the RPi?
Well, I'll bet R-Pi finds a resolution, but it won't be that one. Currently 1920x1200 is the highest resolution supported.
Now, I've asked the question, can the hardware drive higher resolutions at lower refresh rates, and the reply was "yes, in theory". This is just referring to physically driving HDMI. The problem is that there are many other places in the system that this will effect. The memory usage is the obvious one. The ARM will also be producing a much larger image when using X or the browser, so will be that much more sluggish.
I'm not sure we've got a display with that high a resolution in the office, so it may be tricky to test.