This is something I have been looking into recently and there is more and more chatter about the team of 1,000+ content editors in Asia who's sole purpose is to do nothing other than analyse page content. It seems one of the things they are trained to do is to detect purchased links (some of these are very obvious) and flag them up for review. How they do this is something I cannot find clarification on, but there are sometimes ways to see this very easily. If there is actual truth in this (and I suspect there is), tread very carefully because the PR6 site you buy a link from, might do you more damage than good! Andy
I think buying the links won't hurt unless you are not going the link back to the same (or link farm type) sites.
buying links is dangerous, but selling links is more dangerous; because Google is getting more and more smarter