Our N900 can already switch off its single core CPU when idle. I don't think that would change for a dual core. Linux even has a setting that makes it prefer running stuff on one core so that others can sleep.

A few years ago Nokia used "Dual CPU" solutions in most of their symbian handsets. tThen symbian os was improved to the point where it could run well enough on just one cpu, and Nokia went to singlecpu to save costs. "well enough" being a relative term when it comes to symiban, of course