I believe the big draw of DDR3 is that it is lower voltage. 1.5V I believe. Whereas the DDR2 is 1.85V and often run at 1.9 to 2.2V on PCs to eek out a tad more speed...
If you ran 1.5V chips at 1.85V I think you'd ruin them; hence, I think they keyed the slot, etc.
From a response time, I've not seen the DDR3 as specs impressive. Looks like the same number of ns (nano-seconds) to access the memory (higher clock speed * more clocks of waiting = same access time). I think there are some speed tests that only show a 2 or 3% increase for DDR3...
I'm sticking with DDR2 for now.