Update: As pointed out by Aulia Masna, the fine print on the free Find my iPhone instructions makes it clear that the service is only available for the iPhone 4, the iPad or the current (4th generation) iPod touch. 3GS, 3G and original iPhone owners are out of luck; however they can still subscribe to MobileMe and get Find My iPhone as part of the paid service.
There's also a grandfather clause of sorts: if you create your Find My iPhone account on a qualified current-gen device, you can then use that same ID to enable FMI on any iOS 4.2 device, which allows you to stretch back to the 3GS, 3G and previous-gen iPod touch at least.
The ability to remotely locate your iPhone is one of the most appreciated and sought-after features of Apple's MobileMe package. If you've been on a frantic Black Friday shopping spree, for example, and accidentally left your phone behind at an unknown store, you can quickly dial up Find my iPhone on your Mac to figure out where your errant smartphone is hiding.
Once you've upgraded your iPhone or iPad to iOS 4.2, however, you'll be able to use Find My iPhone for free with the iPhone 4, iPad, and 4th Gen iPod Touch.
To activate the feature, go to Settings. Select "Mail, Contacts, Calendars" and choose to Add an Account. Enter your existing Apple ID username and password, which is the same information you use to interact with iTunes.
Free use of Find My iPhone is a significant competitive advantage for iPhones and iPads. It's a handy feature which brings great peace of mind to its users. Hats off to Apple for making it available even outside the MobileMe account.