i'm just getting a hang on Xperia as I switched several devices in few months, but I find this device very hard to develop on.
let me give you an explanation:
-lots of proprietary stuff around stock roms, CM has lots of bugs to be fixed too (especially ICS),
-kernel would would be based on CM, which I explained is buggy, working on half baked software is extremely hard to debug, so it would be best to start fom scratch, which is very time consuming
-baseband and camera interface is nothing like HTC at first sight..could be extremely problematic getting these working, and perhaps never in its full potential (ImageSense is out of question because it's hardware feat)
-SoC is good, same as Xperia arc (not S which is 8255T)
-every sense above version 3 is very hard to port, maybe skinned 2.1 would be better option.
-i'm not counting in any unexpected specifics which may delay development even further.
my toughts: about 10 months with skilled crew of 3 (good kernel developer, one who knows hardware& binary decompiling and one familiar with sense) working their free time and you MIGHT have something worthy to show.
now you may ask yourself if it's really worth it. you are probably better off with selling your device on ebay and buying One V on contract or without one(it's about 300euro in Slovenia off contract)