The process posted by Sharaz22 over at xda-developers is reasonably lengthy and also a bit complicated. To implement root, your handset will need to start off with stock Gingerbread firmware. An exploit is then opened at which point you need to upgrade to ICS to actually activate the exploit. The process only works in ICS and you MUST start the process from Gingerbread. A number of people are reporting success, so if you have been dying to get root on your Xperia S you can find the full instructions here.