One of the interesting things about Mongrel2 is its ability to send output to multiple clients with a single handler message. This has a lot of potential for push applications and while I was investigating Mongrel2 a new version of iOS came out that included changes to Safari. While looking at the list of Safari changes in iOS 4.2 I noticed something called EventSource and went to investigate what it was.
As it turns out EventSource is a newer way of doing browser push currently supported by Chrome, Opera and Safari (mobile Safari as well). There is a good HTML5Rocks post on Server-Sent Events that goes into more detail on the differences of using it over something like WebSockets. One of the differences is that EventSource specially addresses mobile device use with the ability to do a "Connectionless push" through a proxy so the end device can sleep but still receive push notifications.
Before reading on check out my example Mongrel2 ruby handler post if you haven't already. The following examples will be based on the code from that post. I'm also going to use Modernizr to detect support for EventSource so check out my post on using Modernizr to detect browser support as well.