Sony's flash tool restores official ROMs to your unlocked Xperia
Sony has dedicated pages with instructions on easily unlocking your bootloader, and has done well to support the custom ROM community, and the new flash tool is an extension to all of that. Best of all, especially if you're a developer, you'll be able to choose different versions of the Android software if your device supports it, so you can load up Gingerbread or Ice Cream Sandwich if you want to test out an app.
Right now, the hardware that Sony is supporting is pretty limited, so the tool only works with the Xperia S, arc, and arc S, but Sony is planning to expand the compatibility in the future. The flash tool is also supported in many regions, with the notable exceptions of China, North Korea, Iran, Burma, Cuba, Sudan and Syria (which Sony says is due to "distribution issues").