and it's doing notifications, isn't that enough?

If it's not going to sign you out when you switch applications (which it already doesn't), it's not much different than how IMs are handled on a computer except that the IM screen isn't "always up", which I suppose is what people are asking for?

Considering it's a small phone display, I'm okay with it the way it is, though the multitasking bar will help. On the iPad, however, I can see how an always-up IM or Twitter client would be worthwhile.