For HDD internal mechanisms to speed up enough to match the 6 Gb/s max data transfer rate, they would have to go at least four times their current speeds. That means 4+ times on rotation rate, head movement speed, everything. Now consider that on rotation rate the common drives now go 7200 rpm, the "high speed" velociraptors go 10,000 rpm, and there are a few super-speed SCSI units still around that can run at 15,000. It has been this way for almost a decade. Care to project when these mechanisms will achieve over a four times speed-up? Most people think never. Those speeds will be available from entirely different systems that do NOT depend on mechanical motion, such as the SSD's now in the market.
A SATA 6Gb/s mobo make sense becasue it can use any such new faster devices. A SATA 6 Gb/s HDD is ONLY capable of communicating with the mobo controller that fast (max) but will never actually deliver sustained data transfer close to that. The best one might hope for in future HDD's would be to get close to the communication speed capabilities of the SATA II (3 Gb/s) system. So that leads to the question, how much price premium is it worth to buy a SATA 6 Gb/s HDD instead of a SATA II unit? Right now in my view, zero. How long to wait for mechanical HDD's that fast? Don't wait at all!