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/
