If you're planning on using Ubuntu 10.10 Maverick Meerkat on a computer with a processor older than i686, well... you can't.
Ubuntu 10.10 will be dropping support for i386-i586 processors so it can take advantage of the i686 optimizations. Apparently this has been discussed and confirmed at UDS-M.
But there seem to be some issues with this: even some new processors which are especially designed for low power consumption are i586. Such a processor is the Geode LX800 - a pretty new processor for which a bug has been filled on Launchpad but which most probably won't be fixed since Geode LX800 is a i586 processor.
Thanks to Manish for the heads up!