This comprehensive Google+ SEO guide covers every aspect and angle of Google+ and how it impacts search. My normal TL;DR has been replaced with a Google+ SEO Best Practices section located at the bottom of this post.
I will be periodically updating this post (updated January 23, 2012) as things inevitably change. Please don’t hesitate to comment or contact me with observations or when certain features change. I also thank the many people (on Google+) who helped in the research phase of this guide.