This is not just an antivirus. It has a very useful backup mechanism, and for extra protection, it also has a device locating system (named "Missing Device"). Additionally, in the last system, a method for making your device scream is also included, to help you in case that you lost your device and you think it's somewhere near you.
When you install the application, you have to create an account. Later, this account is going to be used in order to manage various tasks, such as: details about scanned and infected files, viewing, managing and sending back to your phone data which has been saved (via backup), locating and making your device scream, and some other tasks.
The entire system works perfectly. It happened for me several times to download infected apps from the Market, and Lookout blocked them. I also tried to locate my device, and it located (not instantly) my phone impressive exactly (it indicated an area of about 50 meters, as when I tried to locate it I was inside a hotel room).
After you create an account, you can enter on https://www.mylookout.com/user/login, from where you can see real-time details about your phone. From here, you can do anything I told you. You can even set how frequent should your device be scanned and backup procedures done.
It works with Android 2.1 and Android 2.2 (tested by me). Search it on the Android Market: "Lookout".