I think that SANs and NAS see their biggest return in Virtualized enviroments. You can build a nice, afforable cluster when you can offload the storage neeeds. But without it, it still has some merits. Since I resell Unitrends, I boast reguarally about the need of network attached storage and disk to disk backup.
The first step most small business would take is to offload thier Exchange data. The second task would be to offload thier file server. These are the two server on which data needs grow the quickest. Then there is Data Protection Manager around the corner. So I give a strong yes, once the price come down a bit more.