Xubuntu runs quite well on Intel machines with 256MB, even to the extent of editing movies. Been there, done that. I'm sure it would be quite acceptable on RasPi if they bothered to port current version to ARM v6. Trouble is, except for RasPi, that's all Android territory.
Xubuntu/Lubuntu/*untu isn't really what Canonical is about though. They're not going to maintain a whole architecture port and the associated autobuild and testing system for something that isn't even their product. Nobody's going to stop some motivated party from porting it, but it doesn't fit Canonical's goals, so they're not going to do it (and maintain it) for nothing. Doubt they'd stop anyone from creating a Pubuntu (hehe), and it's probably not that big a job (relatively) considering the ARM port of Debian is quite mature.
Don't really see the point though. We've got Debian. Ubuntu is just Debian with a more complete default install and a release schedule. hardfp would be nice, but I don't think it'll make too much different for most things, and it'll probably come.