Blackberry has gotten almost there. They are still seperate apps but they integrate well with the hub, good thing is that you don't need to make sure you keep them open, bad thing is that you have to open them to communicate.

Windows phone is very good at this and it also combines conversations with the same person in a single window despite various methods of communication so you can continue conversation over various mediums. Downside is that it doesn't have so many plugins.

The N9 is almost as good as the N900, everything works the same with the bonus that it has a skype icon for those that never delve into the account settings. If it also had skype videocall it would best the N900, but for now the N900 is still the communications' king.