The Computer with no name has passed...

[CoasterMatt]

SATA controllers will address the device at the speed it supports, so it would communicate with the DVD drive at the 1.5gb/s rate and the hard drives at the full 3.0gb/s rate.

According to Wikipedia:

SATA/300's transfer rate is expected to satisfy drive throughput requirements for some time, as the fastest desktop hard disks barely saturate a SATA/150 link. This is why a SATA data cable rated for 1.5 Gbit/s will currently handle second generation, SATA 3.0 Gbit/s sustained and burst data transfers without any loss of performance.

I did some tests with my own system, using SiSoft Sandra and Everest, and even when burning a CD, the HD (a Sata/300 160gb 7200rpm drive) performance didn't drop off during benchmarking.

YMMV

In other news, I've been drafted to build my first 8 core machine - of course, it won't be for my own use, but it'll be fun putting it together.