This is also known as "Rounding Cables", and I have heard of many cases in which ATA 66 cables have been rounded. And have seen speed tests showing no drop in speed. Though I'm sure this is not always the case.

When rounding ATA 66 cables, it is much more common to simply fold them over each other or roll them up, instead of separating the strands. Not only for preventing crosstalk, but also so as not to break the wires inside.

A few companies have caught on to this craze and sell specially made round IDE cables of all speeds. However, sometimes they make them longer to make up for the bunching up of the cables. Longer than 18 inches, and 18 inches is the maximum length for the IDE specification.

Personally, I round my own cables. A 40 strand ribbon cable costs me about $2, and pre-rounded cable about $20. I don't bother rounding 80 strand cables, I don't feel like taking risks.

I wrap all my rounded cables in green electrical tape. It matches the PCB nicely. Now if only I had a window...