Sony Ericsson's custom interface for Android on the X10—and other OSes, like Symbian and Windows Mobile in time-is going to be called Nexus. Like other custom UIs du jour, it'll integrate a bunch of services, like for social networking.
Example: Sony Ericsson Timescape, the "signature" Nexus app, sounds a lot like HTC's Sense in that it pulls in a bunch of services like Facebook and Twitter for your contacts, but it's more straightforwardly organized around timelines, which you can break down by service, events or whatever. There's also an infinite button. For infinite discoveries.
MediaScape looks uncannily like Windows Media Center, if you stuck it on a phone. It's actually nice. The camera software is swanky, with facial recognition and tagging like a real Sony camera.
Apparently, it's not going to be restricted to just smartphones, but could reach some of their more "mainstream" phones. How they're going to get developers to develop apps for Nexus though, is another question entirely.