They may have publicly stated that the Xperia X10 wouldn't be receiving further Android updates just a few months ago, but it looks like Sony Ericsson has had a change of heart: they now say the phone will receive an update to Android 2.3.3 (Gingerbread) sometime in late Q2 or early Q3. You can apparently thank the Xperia PLAY, arc, and neo, as the company says similarities in development of these devices has made it easier to bring the update to the X10.
Unfortunately, there's a catch - the update will only be available for "generic trade kits." In other words:
Today, XperiaTMX10 is available in the market both in generic trade version and in versions customized for operators. Due to the resources available in this special project, we will focus on delivering trade versions of the software. Regarding possible customized operators kit we will have to get back with more information on this at a later stage. But we want to be very clear already at this stage on the fact that some of you may not get this upgrade.
So what does that mean? Well given AT&T's history of updates with Android devices, probably not a lot for those unlucky dogs. Those with branded X10s on other carriers may have things a bit better, but we doubt it. Your best chance to receive the update seems to be if you're rocking an unbranded device, or have rooted and ROMed.
Again, they estimate the update to Android 2.3.3 will be available sometime in late Q2 or early Q3 for unbranded handsets, and will be installed via PC (rather than OTA).
[Source: SE Blog Thanks for the tips, Chuar J. and Ken C.]