This, if Engadget's anonymous tipster is to be believed, is Sony Ericsson's unannounced XPERIA X2, the smartphone that will eventually replace the company's XPERIA X1. Bearing a slide-out QWERTY keyboard, as with its predecessor, and what looks to be a curved profile, the X2 - codenamed "Vulcan" - bears the X Panel shortcut Sony Ericsson use to trigger their panel GUI.
There's also a Windows key on the top left-hand side of the QWERTY keyboard, which confirms that the X2 will use Windows Mobile (and likely version 6.5 of the Microsoft OS). Otherwise everything else is speculation: there appears to be a lens up near the earpiece, which could be a front-facing video camera or might be a light sensor, while the front-panel keys may be touch-sensitive.
The picture is small, and possibly a composite or mockup, but already we're not feeling the design. Compared to the original X1 it looks a little bland and plasticky; hopefully that's a side-effect of the early development stage, and not a sign of things to come.