I read of rumors about a year ago, that they'd cause data corruption. Taken into account that round cables are often bought by tweakers, who overclock a lot... well, data corruption is possible, but I don't believe it's due to cables. It'd be some sort of problem between chair and keyboard.
Just couple of days ago I read a review that had done very thorough performance testing.. with 3 different cables and 2 HD's.
There was your common 45IDE cable, 45cm rounded, and 90cm rounded. 45cm round came at the top in almost every performance test they done, and it was so clear it couldn't be a statistic error.
Of course the graph was a bit misleading, and in gaming or any other heavy duty stuff, you wouldn't know any difference. But still, 45cm rounded was always ahead, followed by the 90cm rounded.