@bec: if IM is important, you might want to wait a bit to see how the new OS improves upon it. Currently IM takes quite awhile to switch back/forth. Everytime the app is started, it takes 2-3 secs to initialize and reconnects to the proxy server to download the changes (all the new msgs) you get while it's backgrounded.

AFAIK the new multitasking implementation does not allow for continuous connection to be maintained in the background, so the main improvement/acceleration that IM apps will gain from OS4 will be just the software initialization time. This affects IM and apps like RSS/Twitters. The latter isn't too problematic, since you don't usually go in/out of those kind of apps anyway.

Of course you can make them run full time in the background with jailbroken tools, but they drain the battery.

PS: Two good IM apps to watch on iPhone: BeeJive and IM+