sem4u - 2:48 pm on Jun 22, 2007 (gmt 0)
Rightly or wrongly Google doesn't want sites to rank because they have paid for a bunch of links. They prefer that links counted in their algorithm have occurred naturally. To combat this Google want site owners to add a 'nofollow' tag to paid links or redirect them. If this happens then advertisers may not want to buy the links as they won't pass PR (unless they give enough traffic to justify the price tag).