To be honest, I think Nokia did a great job optimizing Maemo 5. Perhaps it's just that I've tweaked it, or that I'm running at 950mhz, but compared to a HP TC1100(1ghz x86 processor, 1GB ram, 1024x768 screen) with Ubuntu 10.10, my N900 blows it out of the water as far as responsiveness goes, even tnough the raw power is around half as much.

I think that a proper system would involve something like we have now with the N900+Easy Debian - We have a good mobile UI, and can run desktop applications "seamlessly".
What you'd want to do, though, is set it up with a custom repo of optimized desktop apps - Or at least a set of saner defaults - Touchscreen left/right click is always a problem.