Google after a month or two will detect unnatural links, if there signed up in GWT they will get the message:
We've detected that some of your site's pages may be using techniques that are outside Google's Webmaster Guidelines.
Specifically, look for possibly artificial or unnatural links pointing to your site that could be intended to manipulate PageRank. Examples of unnatural linking could include buying links to pass PageRank or participating in link schemes.
We encourage you to make changes to your site so that it meets our quality guidelines. Once you've made these changes, please submit your site for reconsideration in Google's search results.
If you find unnatural links to your site that you are unable to control or remove, please provide the details in your reconsideration request.
If you have any questions about how to resolve this issue, please see our Webmaster Help Forum for support.
Google Search Quality Team
To get out of this penalty the site owner will have to remove the links or be able to explain what happened, and in most cases the verdict will not side with the site owner.
These are just my thoughts based on what I have personally seen.
Also read this article http://searchengineland.com/google-sent-over-700000-messages-via-webmaster-tools-in-past-two-months-113807 we have been seeing a lot of posts regarding penalty's for links showing up in GWT, and I have gotten 2 of these this month myself on old established sites.
Edited by Catalyst, 18 March 2012 - 05:04 PM.