You know Ubuntu mobile is not Ubuntu, hence not a "standard mainstream distro", and uses Hildon, right?
.... Well, I hope you saw the noBounds video, right? That's going to be killer on the N900 (step 4/5); you're on the money with what needs to happen, but Nokia's working on it already. It's unimaginable that the N900 won't have at least twice the RAM; I'd guess 4x; we've already got dynamic CPU frequency, to a max of 400 MHZ; OOo is still slow... But the N900 will get around double the maximum performance, so yes, that'll work. But remember, OOo will violate the "should focus on how one interacts with a handheld device, not how to cut pixels to fit a desktop environment on a handheld device."; that's always a tradeoff, and OOo won't (IMHO) be possible to convert to a nice hand-held app. Still, once the performance is there, I'm sure it'll be available for all who want it, regardless of the OS.
Yeah, and you'll get it back; they've heard, and they're working. Just don't write it off for good, because the best is yet to come. Well, in classic backwards fashion, I spent most of the first 80% of these comments answering your last question... Enjoy!