Patrick Gavin, CEO of Text Links Ads and a regular speaker at SES, has put together a great starter kit for people new to buying text links.
During the recent SES in San Jose he was on the Linking Buying panel and gave some very good advise that most of his fellow presenters echoed. "Don't buy links just for the inbound text link."
Granted there had to have been some Google people there monitoring this panel as there have been in the past. Some suggested Matt Cutts was their watching who attended - but that seems to be the running joke of this repeated panel.
If you buy inbound links for their advertising the extra push they may give to your organic results are the bonus. This is not going to change - the alt tag text of banner ads when you can get them has a similiar if slightly lesser impact on organic.
This introductory primer is a great start for anyone consdiering the purchase of links.... good work guys!