Like Alycia, I had trouble finding a single source on police dissent. Below, I have a bunch of articles and some academic texts that I hope are helpful.

In addition to the examples that Alycia provided, there’s also a famous and recent incident with a retired Philadelphia police chief Ray Lewis protesting at OWS:
http://philadelphia.cbslocal.com/2011/12/08/retired-philly-police-captain-arrested-in-ny-occupy-wall-street-rally-warned/

This is an article that interviews Lewis, and documents one other Oakland police officer on the record as supporting OWS:
http://rt.com/news/police-join-protesters-ows-621/

These were the only references I could find to police joining OWS protests, but I thought I’d also point you to an article about Michael Moore calling for police to join OWS
http://newsbusters.org/blogs/noel-sheppard/2011/10/15/michael-moore-calls-police-join-occupy-wall-street-protests-egyptian

In terms of what I could find about police and protesting, there are two main strains - whiste blowing on police corruption and protesting against working conditions.
About the former, this is a legal article on what makes whistle blowing difficult in police culture and why it’s necessary:
http://www.bmartin.cc/dissent/documents/Johnson.pdf
Here’s another academic article on obstacles to whistle blowing among police:
http://cad.sagepub.com/content/53/4/605.abstract

And here are some news articles on different instances of whistle blowing in various police departments:
http://blog.oregonlive.com/oregonianextra/2008/05/whistleblowing_lawsuit_former.html
http://www.baconalert.com/2011/07/13/newark-police-officer-allegedly-fired-for-whistle-blowing-on-corrupt-cops-newark-new-jersey/

You might have already heard about these, but the most famous police whistle blower is Frank Serpico, whose biography is here
http://www.worldcat.org/oclc/621098
and the Al Pacino movie is here
http://www.imdb.com/title/tt0070666/

As far as what seems to get police to protest, it looks like mostly working conditions. So here’s an article of Paterson police protesting layoffs
http://www.nj.com/news/index.ssf/2011/04/paterson_police_protest_125_la.html
Sydney police protesting benefits cuts
http://www.salon.com/2011/11/22/3000_police_protest_in_sydney_over_benefit_change/
Italian police protesting budget cuts
http://www.infowars.com/police-revolt-italian-police-protest-austerity-cuts/

Of course, police are unionized, so their decisions to protest are largely going to be based on the unions’ stances towards the protest. The Wisconsin protest last winter seem to have garnered some police support:
http://understory.ran.org/2011/02/25/breaking-wisconsin-police-have-joined-protest-inside-state-capitol/#
http://rt.com/news/police-join-protesters-ows-621/
http://www.usatoday.com/news/nation/2011-02-26-wisconsin-officers_N.htm

So there's some hope for dissent based on the Wisconsin protests, plus maybe also global incidents of police support for shared economic hardship.