My opinion on this is that the FC SAN (and block storage in general) as we know it will begin to fade (or at least not continue to grow at the rate we are currently seeing). The real reason it will fade is not because it's going to be superseded by FCoE, or by NAS, or by iSCSI, but because applications will likely move to an object-storage model. When we start referencing the documents that we save in MS Word, Excel, PowerPoint, and the likes as an object on an object store (either in our home or in the cloud) the need for an FC SAN diminishes. There will still be workloads that live on an FC SAN forever due to its isolation, due to its speed, and due to its resiliency. My cat photos, home videos, resume's, and rambling dissertations on life don't need to be stored on an FC SAN in the cloud. They will just as happily live on object storage that has as much (or more) data resiliency as an FC SAN, but a fraction of the speed and price.
I see this trend starting now, and I would expect to see a large shift in the next 8-15 years. What does that mean to me? FC SAN has at least two generations of releases before it's relegated to the high-performance workloads that it was originally designed to handle. These workloads will likely include transactional databases, Email servers, and other legacy services consuming block device resources.