IM+ offers IM Push on the iPhone already
In response to what they call the unreliable Push Notification system in Windows Phone 7, Shape Services are planning to bring their IM Push system, which sends IM messages via email to poorly connected devices on the Blackberry and iPhone, to Windows Phone 7 also.
In the interim they suggest users who are not receiving their push notifications to either try a different network, as some networks block push messages, or to try rebooting their device.
They do however note they expect the situation to improve in Mango, but I am sure another channel for reliable IM messaging will not go amiss even then.
For our many readers on Mango already, have you seen an improvement in Push Notification reliability? Let us know below.