So my question is not "if" but "how". How can I (on PC) play with PS3 players, or vice-versa?
I don't believe this is actually currently supported. It's listed as a feature, but it's also listed as "coming soon."
In a reply to a forum thread, one of the developers of the game stated:
This was the freshest data I could get, and it's dated November of 2011.
They've also mentioned that patching the console versions is a costly process ($10,000 according to other replies from the devs in that thread).
It's hard to say when the support will be there, as that's speculative and not really authoritatively answerable.