In How to get a phD book, the authors did recommend that the thesis be written as soon as possible. This is to avoid being stuck during the last part of phD duration and having problems such as writer's block etc. That is a very good advice, seemed logical and could prevent any catastrophic incident during 3rd year phD.
My supervisor really fond of that idea and had asked me to give him the skeleton draft of my thesis in my 2nd week being here....
It was quite hard to start because I don't know where to start, what kind of format etc. So I asked fellow phD student for a copy and I found several phD thesis lying around in my office.
I followed the format etc. and handed it over to my supervisor. It was as what he wanted, a bare skeleton draft.
But I think it worth doing in early stages of our phD study. Since then, the thesis is developing as I keep inserting what I read for my lit review etc.
So start writing early. Write what you read and keep that piling up :)