Why? SATA II is capable of up to 300MB/s transfer speed. Gigabit LAN theoretical max is 125MB/s. You'll never achieve 125MB/s in real world use.
You would need to be on a 10GigE LAN to exceed the speed that SATA II disks could handle. So, don't hold your breath waiting for consumer/prosumer NAS boxes to support SATA III.
IMO, a more important and valid question is when will they support USB 3??? Another $1M question. My guess on this is you'll begin to see NAS boxes with USB 3 before the end of Q3 2010.