Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_2_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148 Safari/6533.18.5)
I use an app called
Verbs. it supports google talk and AIM/MobileMe. It's worked really well for me so far. You get local notifications if you're using an iPhone 3GS or 4 - you stay logged in for 10mins in the background. No push notifications yet, but they're coming in an update soon apparently.