Remember the Droid hack for making a USB host? Well, I guess that hack can work for most Android-based smartphones plus any regular USB keyboard will be recognized if you simply plug it in.
Android sees the keyboard, and as Chris says, “It Just Works.” A similar process was replicated and tested successfully on an HTC Hero as well, so odds are decent that many other Android phones can use USB-OTG to host a keyboard. No additional drivers were needed for either the Droid or the Hero, so keyboard support has to be built within Android itself. Of course, that means it could go away in any future Android updates, but for now, you can enjoy a full-sized keyboard on your pint-sized phone.
I think Google has done a great job of implementing USB devices on the Android as it’s not just a smartphone OS, it can be extended as any kind of control device for many gadgets.