Whitey - 1:05 am on Jul 5, 2011 (gmt 0)
... it will mean [that someone is] buying links [pointing to your site].
If this takes off in WMT it provides an opportunity to communicate with Google and take action. Competitors probably don't want to invest heavily in ruining another site, with the risk of it backfiring. It's not difficult to find out who paid for those links and enforce remedies in theory.
Google has detected unatural links and link patterns for many years and penalised sites accordingly. Their difficulty has always been in fully automating it, often having to rely on hand reviews and spam reports probably on trophy term and traffic reports.
Any high trafficked, trophy term driven site, or network of sites is a target for flagging and review.
The fact that notifications are ramping up in WMT is a sign of Google becoming more confident about warning webmasters openly, and no doubt their detection methods are getting better. It's not difficult to work out the more obvious methods.
The day's of relying on paid links alone as a main game are long gone. One or two , maybe , but why bother if sustainability is important for your site.