Everything you need to know about SEO, delivered every Thursday.
Google Claims To Investigate All Authenticated Spam Reports
A Google Webmaster Central Blog post on spam reporting claims that all authenticated spam reports are reviewed by a Google representative.
An authenticated spam report is a spam report filed within the Google Webmaster Central tools section. Spam reports filed via the unauthenticated spam form are not all reviewed, but Google says “a large fraction of those are reviewed.”