Web filters - Effects on Productivity

I'm currently researching web filtering.  While my job is not to play
Internet Police with those whom I work, I do find it interesting that
even though my company has a seemingly draconian Internet Access
Policy, people still seem to waste plenty of time on the Internet.  I,
for one, am not exempt from this statement.

My thoughts are that Web Filtering means different things to different
people.  HR believes that blocking sites such as CareerBuilder,
Monster and LinkedIn keeps employees from sniffing out better jobs on
company time.  Desktop support believes it decreases the number of
avenues for a widespread virus outbreak.  Managers believe it keeps
employees from wasting time in chat and on social networking sites.

At the end of the day, I'm not sure how effective the productivity
piece really is.  If users can't access Facebook many settle for
wasting an hour on the "letters to the editor" section of the local
newspaper's website.  As for the blogs I read (many security-related
blogs are hosted on Blogger or Wordpress, which are blocked), I simply
add them to Google Reader, which I can access.  I read them anyways,
at least they're related to my job.

I'm not bringing into question the technical security benefits of web
filtering; those are obvious.  Do web filters in schools and offices
*really* give productivity a boost, or do they simply shift what sites
or activities employees waste company time on?  Have there been any
solid studies on this topic?

--Noah