There 'could' be a market for it on ARM devices though. It could have a niche in between the alternatives. It has plenty of games and apps to bring across with it.
Being a user of RiscOS (still having an RiscPC running here...) I must admit that even the port of a *native* (!) desktop operating system like RiscOS to recent ARM hardware like the Beagle board or maybe the recent stuff from Genesi is a task that really, really laaaaaasts! I wouldn't assume that a port of an "ARM-alien" OS like AmigaOS or MorphOS will be really faster... (except you have some spare millions for a bunch of coders... ;-))