Arjen, you are absolutely right.  It doesn't matter how over-engineered a storage solution is (I'm thinking of a giant dual-headed Netapp with redundant everything).  After you've paid a few hundred K for that, you still have a single point of failure.  Is it a highly-unlikely point of failure?  Sure, but it's still a point of failure. 

Let's take it a step further, at Yahoo we're beyond thinking about how to make a single node redundant (be it for storage, networking, or even a simple webserver), we consider entire datacenters to be single points of failure.  What does that mean?