If a large part of your business model deals with having advertisers buy ads off of your site you might be in for a big suprise a little later in life.
http://www.mattcutts.com/blog/how-to-report-paid-links/
In a nutshell Matt is asking folks to report paid link buyers and sellers. Yep – black, grey, and whit hat seo’s are quacking in their boots. Mainly over this line:
Chaaban, nope, definitely not an April Fool’s joke.
We’ve got a lot of data within Google already, but I wanted to put out a call for external reports to widen the set of data that we can test on.
In other words kids – Goog is already on the hunt for link farms – we know that… but they are also now on the hunt for those of you who buy and sell links with out using no_follow tags. Read through the comments a bit (there is over 100 of them), and form your own opinion.
Then go out and do no evil 
Allow me to be the first to comment
Google won’t definitely do that. Why? Because there will be tons of reports on links farm from people even don’t know what link farms is
How could Google do that? Google even can’t do anything to drive old schooler blackhatters off the high rank list
You are kidding right? Google WILL most likely do that. They do get rid of the blackhatters over time – they just make more sites. Get them blacklisted – make more sites, etc…
Never ending cycle.
Google won’t bann you for linkfarming, but they may very well penalize you.
Read the comments on Cutt’s blog – the writing is probably on the wall.
To follow up here:
Webmasters are reporting that if they sell sitewide links or have been buying sitewide links they have a huge pr drop.
It happened.