What's the best way to find a product management role on an open source software project?

Open source projects work on a very simple principle: code talks, bullshit walks.

Make contributions to the technical side of the project—if you don’t, you’ll be written off as a wannabe. File bugs, diagnose them, fix them. Participate in mailing lists. Show that you understand where the project needs to go and what needs to be done to get there. Eventually you’ll be working in a product management role without the job title.

As a current product manager, my advice would be to: Don’t give up. Really this job and for most product managers, has an experience of all guts no glory. And moving from a development background to a product manager is difficult – I hope you are not running a scrum team…if you are find a happy balance or move to a different company.
For me personally I read and listen to webinars by pragmatic marketing as well as industry webinars to help me keep my chin up and not give up on this career path, as well as find new industries to work in.
I’ve included a few links that I hope you find helpful.
http://www.pragmaticmarketing.com/pragmatic-marketing-framework/?searchterm=framework
http://onproductmanagement.wordpress.com/
http://writethatdown.com/
http://www.goodproductmanager.com/
Just to answer your question further, for every company I’ve worked for, the product management experience/culture has been different. Some want me to only be technical and offer no advice on user experience while others want me to run the gambit and blame me for it as well. What I do to keep myself growing professionally is I volunteer. I do research for companies and write business cases as well as requirements for free. Of course these are non-profits and at the end of the day it’s good karma, but I’ve found it to be a good way to make up for all the existing dismissal of the importance of product management that are in most companies; and it helps on the resume. In the mean time – for the money making part – find a big company to work for with an existing product management mentor, they’ll fill in the gaps for building your credibility with a company outside of a compiler. As a warning I’ve never had luck with small companies – there seems to be more confusion/chaos than actual business. Sorry if this is long winded! Good luck with your search!