the HTC universal was a brilliant piece of hardware, and at one point looked as if it was going to take over from the Zaurus as a pocket computer with keyboard as the Uni had wifi and bluetooth and phone features, when the Zaurus only had the CF slot.

The Uni came with Windows Mobile and even though there was almost no documentation people found a way to make it run linux. Progress was painful, but they managed pretty well.
http://handhelds.org/moin/moin.cgi/Universal
Unfortunately by the time linux ran fairly well on it, the world was moving on and the universal was dropped from HTC's line and Android was imminent (very rough time lines).

So, yes, in theory it can be done, but it's very hard to get linux to run really well on a device so that all its hardware is supported... and quite often when the device's makers realise their device is under scrutiny, lock it down even more - see ipod linux projects.