This is really awesome; except as an app developer, I really rather get a library that I own and have on my own infrastructure than a SaaS model, especially for something so critical to my application.
If you just want the raw "back end WebSocket-compatible multiplexing daemon" functionality, that's not hard to code/devise. You can use something like: http://github.com/gimite/web-socket-ruby - What services like Pusher provide is all the API stuff on the front, maintaining the servers, keeping those daemons running, and, well, just making it super easy to get going without worrying about the back end so much.
All the API stuff on the front I'd pay for. However, I'm already paying for "maintaining servers, keeping them running, and worrying about the backend" -- so I don't want to pay for it again.
This might just be MY preference, and others might beg to differ.