So, passive mentoring, to me, is learning from someone else without them purposly trying to teach you.
For a company to participate in passive mentoring, they need to find a company that they feel has strengths that they don't, and learn from that company how to build those strengths.
My company of choice to act as a mentor is a small company named 37 Signals. They have a small staff which includes a mere 4 programmers, and 3 interface designers. Even with this small size they have attracted the creator of Ruby on Rails, and get tons of free publicity. Some of this includes an interview with Jason Fried of 37 Signals by The Web 2.0 Show, and various mentions on other podcasts including one about their blog and upcoming Vitamin site.
If you haven't heard of them, I suggest that you check them out. I think that they have a lot to teach.