Robert_Charlton - 6:38 am on Jun 6, 2008 (gmt 0)

Why would Google treat Yahoo different than any other site selling paid links?

Because Yahoo charges for reviewing your site, and the charge doesn't carry a guarantee that they'll include you. They also have certain editorial criteria that are fairly strictly adhered to, certain more than on most free directories or paid link networks.

That human review is in itself a measure of quality which I believe helped Google's search quality a lot when Google started out, seeded as it was with Yahoo Directory pages.

It's generally been assumed that inclusion in Yahoo still confers "trust" of some sort on listed sites.