Sorry I was a bit off in my description of things. Blackberry push is a proprietary technology so any other method is only trying to mimic their push feature. Activesync doesn't actually fetch in small intervals but keeps a connection open between the server and the device in certain intervals to allow the server to tell the device there is a notification, then the device will go out and pull in that message.
In a BB world the server doesn't tell the device it has a message it just gives the message to the device. The BB is passive in the process. The iPhone actually pulls down the email after receiving notification of a message.
In the end the difference is probably only seconds, but the BB method is the only real true push out there, activesync just gets incredibly close to true push.
This is why turning push on with the iPhone reduces battery life and has no impact on a blackberry.
I can only assume mobileme also operates in the same fashion as activesync but I could be wrong.
I suppose this is kind of off topic though so I appologize.
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