i heard there was a small run of xo´s with xp made by olpc following a paif order of microsoft 8 1 or 2 k units).
if one could read out the pic 7 eprom containing the efi / bios info and had a soldering gun to put a accordingly modified pic of digikey on the xo motherboard instead the oem one xp should boot nicely.
other than that onyl a small linux bootloader and any flavour linux that can run well with limitee system resources should do, even more the versions that where tailored around the xo´s display and gamekey features.
once the gen 1.5 with via chipset hits the shelves power management and even more advanced graphics modes should be easily integratable with evenmore distro´s not solely "extraordinary" or modded ubuntus and alike.
i wonder if moblin or android will work with the xo lateron