If I'm not mistaken IM clients still work similar to how they are on OS3.x. When you minimize the app, you get updates through push notification and the IM app doesn't maintain continuous connection to the server. When you switch back to the IM app, then it'll reestablish connection and download all the updates that you've missed. There'll be speed increase when switching in/out of IM apps due to them not needing to be initialized everytime you run them, but there'll still be a pause as it reconnects to the server-side daemons and downloads all the missed messages.
Another good example is Skype: In previous versions, Skype needs to be open to receive calls. That makes it useless as a phone replacement. In iPhone OS 4, Skype can keep running in the background to receive calls. It doesn't matter you are surfing the web or playing a game: You will be able to pick up the call—switching instantly to Skype—have a conversation for as long as you want, and then return to your browser or game just where you left it.