Interesting approach from Sun. Being "Big Tin" manufacturers and wholesalers they have a rather different take on the Flash vs HD argument. Simply put Flash uses less power, has better bandwidth, much, much, much better latency and is in general more reliable BUT billions of writes could wear them out. Hard drives have WAY more storage per dollar/euro/pound/yen BUT there are many components that could fail.
So... which is cheaper? Sun don't view this is a simple $/Gb... They see it as "dollars per gigabyte served". Once you take into account power things change dramatically.
There's also talk of why ZFS fits this well (in a nutshell, ZFS is very good at spotting and routing around errors
Sun CEO Jonathan Schwartz Blog Entry "Anything But a Flash in the Pan"