Sony Ericsson just proved to us all that they get it. First, they offer up a real bootloader unlocking solution, and now they have published a How-To on their official developer blog with instructions to build, flash and boot a custom Linux kernel for the Xperia Arc, the Xperia Play, and the Xperia Neo. They even provide links to the needed tools, and give some tips for editing the kernel source and config.
To all the Android kernel developers out there this is like solid gold. No more fighting the manufacturer and their protection schemes, instead they provide a How-To and offer support at XDA-Developers. If only other manufacturers would follow their lead.
Source: Sony Ericsson Developer Blog