Question: What are the differences between NowJS and Pusher?

Answer: 
As an addendum, here's RMSN - a drop in replacement for Pusher that I've written using NowJS.
<https://github.com/leppert/RMSN>
There are naturally lots of differences, but mainly:

Pusher is a framework that can be used with several platforms (Java, Python, PHP, Ruby, and lots more) and uses a centralized server at pusher.com with your account that handles the Comet-technology stack.

NowJS is all node and runs on your own server.