In tablets, having buttons on-screen rather than built into the body is standard operating procedure. On phones, however, there's really only one that eschews physical buttons for software, and that's the Galaxy Nexus. Still fuzzy on what I mean? Take a look at the comparison shot below, lifted from our review of the Galaxy Nexus:
On the left, the Galaxy Nexus, with its three software buttons displayed on-screen. On the right, a Motorola Atrix, with its four hardware buttons at the bottom.