The USPTO now has a landing page devoted to third-party preissuance submissions: http://www.uspto.gov/patents/init_events/preissuance_submissions.jsp
The October 2012 issue of Inventors Eye explains that a preissuance submission "allows any member of the public (a third party) the option of presenting
the United States Patent and Trademark Office (USPTO) with information
that may be relevant in determining the patentability of a specific
application." This new page "features statistics, lessons learned, and frequently asked questions, among many other topics." Since the preissuance submission provision went into effect on September 16, 2012, the USPTO has received nearly 1,500 submissions.