Finally after the launch of OS 3.0 for the iPhone, the Application IM+ updates to Version 3.1. push notifications are not done via email anymore, they now use Apples push notification service.
I tested the delay with a script, which sends me jabber messages. The scripts sends out a message every five seconds to my Admium desktop application. It takes about the same time to the IM+ app, when it is open. If the message goes via the push notification service the delay goes up to approx. 10 seconds.