I haven’t received my new MacBook Pro for testing yet, but a reliable source has confirmed that only the standard drive bay utilizes SATA III 6 Gb/sec whereas the optical bay still utilizes SATA II 3 Gb/sec. That’s too bad, but unlikely to matter for any real world situation, it’s more bragging rights than anything else.
A RAID-0 stripe of two internal drives is still possible as with an MPG Pro Laptop, but will be closer to the limits of 2 X 3 Gb/sec SATA, or about 600MB/sec with two SSDs, probably more like 550MB/sec in practice.
A RAID-1 mirror for fault tolerance is also a possibility with dual internal drives. And once Thunderbolt-based drives appear this summer, something well over 1000MB/sec should be possible.