I've definitely known/heard of a lot of cats that do this... I think the porceline, marble, etc is probably cool (keep in mind, cats' natural normal body temps are above 100*F) and the shape makes for an easy curl up space.... I can't really think off the top of my head why this would be a bad problem, other than restricting the human ability to wash hands/brush teeth/use the sink at all, hahaha

but since when do our kitties care when they are in our way?! haha Fortunately, Oliver can't get into my kitchen sink (he knows he's not allowed on the counters and there's not enough run space to get his 15 pound bum up there anyhow) and the only time he hops in the bathroom sink is to lick the drips in the drain.... again, he'd prolly be too big to fit in that sink anyhow, haha