Heaped salt at the ready, but this could well be Sony Ericsson's upcoming refresh to the XPERIA X10. Said to be the Sony Ericsson ANZU, the image was passed to XperiaX10 with scant technical details aside from a 4.3-inch display and an HDMI output.
The screenshot itself indicates the ANZU is running Android 2.1, which we're hoping is a mid-development thing rather than an indicator of what version will actually ship. According to the source, the handset is "very, very slim" and Sony Ericsson is aiming for a Q1 2011 release.
Update: More pictures from XperiaX10Blog added of what's being cautiously called the Sony Ericsson XPERIA X12.