Find my iPhone and Find My iPad have been around for awhile via MobileMe, and we've heard dozens of success stories where people used the service to recover lost or stolen iOS devices. Macs have been reliant on third-party solutions or clever hacking to enable the same functionality -- until now.
With iCloud comes Find My Mac, a feature that's essentially identical to what's been offered for iOS devices. If you have a free iCloud account and your Mac is running OS X Lion 10.7.2 or greater, has Wi-Fi access, has a recovery partition installed (more on that later) and has been set up beforehand in the iCloud preferences, you can use Find My Mac to locate your computer anywhere in the world using either iCloud.com or the Find My iPhone app on an iOS device.
Find My Mac is not enabled by default (possibly out of privacy concerns), so once you've met all the relevant prerequisites, go into System Preferences on your Mac, navigate to the iCloud preference pane, and tick the checkbox next to Find My Mac to enable it.
If you've used Find My iPhone before, you'll be familiar with how Find My Mac works. You can locate your device on a map with a fairly reasonable degree of precision, assuming it's connected to Wi-Fi. You can also "ping" it so that a message of your choosing will pop up on screen (along with a very loud sonar sound alert), or you can remote lock or remote wipe the device. Remote wiping the Mac should be a last resort, though, because after you pull that trigger you won't be able to use Find My Mac to locate it. If Find My Mac can't find your device right away, you can even tell the service to send you an email when it does finally locate your Mac. It's all quite slick, and it should bring peace of mind to MacBook Air and Pro owners in particular.
Find My Mac requires your Mac to have a recovery partition installed on it in order for the service to work. If you downloaded OS X Lion from the Mac App Store and did a normal installation, you already have a recovery partition on your Mac, so you don't need to worry about that requirement. But if you're like me and you used a third-party app to clone your Lion installation from an old disk to a new one, you likely missed out on that recovery partition and won't be able to take advantage of Find My Mac until you install one. There are various hacky solutions to this problem, none of them particularly user-friendly, but I found it easiest to simply re-install Lion completely and start fresh.
Since Macs don't have built-in 3G or persistent Wi-Fi when they're asleep (in most cases), iCloud is going to have a harder time locating stolen Macs than iPhones or iPads. It's still a very handy feature, and it significantly increases your chances of recovering a Mac that's mysteriously sprouted legs and gone walkabout.