If you don't mind a short warranty and a reconditioned (though largely like-new) unit, Buy.com has the deal to beat on a silver 4GB iPod nano.
An iPod nano for under $100?! Sign me up. Buy.com has the 4GB silver model for $99 shipped. No rebates, either! So what's the catch? These are reconditioned units with a 60-day warranty. If that doesn't square with you, the Apple store currently has the same refurb nano for $129 with a 1-year warranty.
Of course, nanos have few mechanical parts, so there's little opportunity for something to break. I do wish you could choose from colors other than silver, but that's a minor quibble. The 3rd-gen nano is a wafer-thin thing of beauty, and an outright steal at $99.