I recently upgraded to ICS on my Xperia Ray (since I understand that Sony will not be upgrading my SI number) and thought I would share the experience in a document. I have purposefully avoided the word "guide" (except for in the subject of this post) as the document is purely based on my experience of flashing the ICS ROM to my phone. Basically I felt overwhelmed by all the information and just wanted a step-by-step guide - this was my inspiration for writing up my personal experience.
Ironically, when re-visiting this forum to post this message, I came across this great post by Nabeel, which I wish I had found before I started! I guess I was being too specific in my searches for information, always searching for "Xperia Ray"...
Anyway, I don't wish to tread on anybody's toes with this, and maybe it is just duplicating what others have said, but I have spent so much time in preparing this document that it would seem wasteful not to share it now.
https://docs.google.com/document/pub?id=1wU05V69SlTMrKNLVMPJvKvYmA9qOA1NU4G62P2TFiww
In addition, I have described my experience with rooting. It's fairly simple to do, but if you try to be too clever, like I did, then you may trip yourself up!