I'm using Beejive and it seems to be working on iOS4.0 with the most recent update - push notifications are the way we're being alerted though, and it definitely looks like fast-app switching is there, saving the state of conversations etc to return to.
It would be nice though if they did confirm it was 4.0 supported and what if any multi-tasking was supported.
It works in 4, sure, but mine does not save state. It has to launch and load each time I switch over to it. In fact, no IM app has updated to work with multitasking yet and it's annoying me to no end.