There comes time when trying to support older hardware can't be
reconciled with advancing the OS, and that applies to all OS's not
just Ubuntu. If that didn't happen we'd all still be running 6502's or
Z80's with 8k RAM

I'm sure that there'll be other versions of Linux that will run quite
happily on lower spec hardware, even Ubuntu ones - though these won't
be from Canonical who've clearly got their sights on bigger fish.

Time to go looking but 10.04LTS is still supported until April 2013. I
don't think there's any mileage in trying to force the issue on older
hardware that's just not up to the task.