I think it's too late for that. Perhaps a few years back it might have had a good chance of developing a niche for the kind of people happy to play around with / hack their mobile devices. I completely agree with Jaffa's following comment, especially the last sentence:
"RISC OS would be a cool distraction on such a device but it'd be useless for getting any real work done, and you'd lose so many of the features and gain very little. The expectations of a mobile device already exceed what RISC OS can do."
Now, I think the war is on in the Mobile OS arena. There is no market leader yet and it's growing very fast.
Indeed, Windows Mobile is crap. But it's Microsoft and we all know they're not backing down until they own 80% or more. In my opinion Apple's iPhone and iPod touch devices have raised the bar significantly regarding interface design and usability on mobile devices. With the official release of a SDK for these devices next month, they're going to sell like hot cakes. In the US, the iPhone already has a larger marketshare than Windows Mobile and it's not even been out for a year there.