What are you talking about? What functionality does an instant message application need that isn't supported? It can get push notifications if someone sends you a message. You could even leave your app running for up to 10 minutes in the background waiting for updates if you wanted (which probably matches actual usage pretty well).
What you can't do is have your app sitting in the wait queue for data coming in over the wifi because that would mean the wifi card has to run constantly. Ask Android how quickly doing that can drain your battery.
You get AOL to add support for apple's push server into their Oscar protocol chain and I'll concede.
My battery lasts fine and I'm generally logged into IRC and/or meebo for most of the day in addition to the Google Talk that I leave signed in and my three email accounts that get updates. etc, etc. The battery woes are simply unjustified for the most part.
I couldn't find the article in 30 seconds, but I recall someone from Google, Android or one of the carriers (i.e. someone qualified to talk about it) who acknowledged the battery issue and pointed to exactly this as the culprit.
What you can't do is have your app sitting in the wait queue for data coming in over the wifi because that would mean the wifi card has to run constantly. Ask Android how quickly doing that can drain your battery.