As a Protoss how do I defend against the Stefano style build that consists of making three very early bases and pushing once there is a huge number of roaches, sometimes maxed out, at a very early point in the game? It is very deadly to protoss and I don't know how they can even think of stopping it.
If you mean the 11-minute max-roach thing, the latest Day daily covers exactly that: http://day9.tv/d9d472/
It basically boils down to lots of sentries to slice his army into manageable pieces.
Your core units to defend it are Sentries and Immortals which must be kept alive at any cost. You also need to keep your third alive which is the hardest part since the attack comes right when you are about to really establish it.
Basically you wait for the opponent to come through any choke point near the natural or the third, forcefield ~1/3 of his army and crush it with your stalker/immortal forces. Then you forcefield the next chunk until you max out on stalkers and immortals and then you just go kill him.
Void Rays are another option that MC seems to prefer but it is a lot riskier since he can drown you in more roaches than your Void Rays can kill in time, which is also called the Mondragon-Counter.
The three base roach attack wins by shutting down your third, then killing you. If you haven't got a third, but have a highly defensible natural/main and don't move out until you are 3/3 with a 200/200 death ball you should be able to beat it. A maxed Collossus/Archon/Immortal/Sentry/Stalker army does hideous damage to roach/ling maxed armies. Roaches are very supply inefficient.
Remember that he's spending all his gas on roaches, he won't have a spire or infestation pit and so getting high level tech is going to take a while.