This is really cool. To make sure I understand, using it would replace the need to use services like pusher.com? Also, if I were using, say, Sinatra with nginx and unicorn, where would this sit in the pipeline?
To be clear, this is software, so comparing it to Pusher.com is a bit of an apples to oranges comparison. Pushpin is more comparable to Socket.io, Juggernaut, Faye, etc.
What makes Pushpin special is that you can control what the outside-facing HTTP exchanges look like, which makes it good for implementing APIs, and may also be useful if you're just really anal about how your client/server interactions work. :)
The cost is that you need to design your protocol and write client code (the other solutions already have their own special protocols and come with corresponding JavaScript libraries ready to go). So whether or not Pushpin is good for you depends on what level of control you're after.
In the pipeline, Pushpin goes in the very front, just behind a load balancer (if any). The reason for this is you could put instances of Pushpin in different geographic locations, all fronting an application in a single location. So you want it the furthest out, closest to any users that might be connected to it.
