From Bob Cringely's (whom I love - not literally) latest spouting:

“Microsoft is woefully late with its next Windows upgrade, while Apple is far ahead with even the current version of OS X. Apple is moving to Intel processors and hackers have already shown that OS X can run fine on non-Apple hardware. But Apple doesn't want to give up its profitable hardware business to compete head-to-head with Microsoft. And remember, Apple totally dominates the portable music player market and will probably sell 25 million iPods or more this year.

Every one of those iPods is a bootable drive. What if Apple introduces OS 10.5, its next super-duper operating system release, and at the same time starts loading FOR FREE the current operating system version -- OS 10.4 -- on every new iPod in a version that runs on generic Intel boxes? What if they also make 10.4 a free download through the iTunes Music Store?

It wouldn't kill Microsoft, but it would hurt the company, both emotionally and materially. And it wouldn't hurt Apple at all. Apple hardware sales would be driven by OS 10.5 and all giving away 10.4 would do is help sell more iPods and attract more customers to Apple's store.”

As far as I'm aware, you can already boot and run OS X from an iPod today, but obviously require a Mac from which to boot and actually run OS X, the iPod merely acting as a hard disk - which isn't strictly the same as what Cringely is talking about, but I can clearly picture Steve Jobs pulling a Dr. Evil pinky pose right now.