To an extent filer clusters are SAN's. I personally think NAS is superior to SANs architecturally, even if NAS's use SAN's.
One day we'll have a network for network traffic, another for disk traffic, another for backup traffic, another for montoring/maintenance traffic, and some bright spark will come along and say 'We could do all this with one proper network!' and I'll just shake my head and cry.
8)