This year at WWDC 2011 Apple announced iMessage, a BBM-like instant messaging service built right into the SMS/MMS app. They also built Twitter right into the OS, for clip sharing in Safari, Photos, etc. and come this fall, authentication for the official Twitter app and others. Last year at WWDC 2010 they introduced FaceTime, a video calling service that existed first in the Phone app, then later in its own app on iPod touch, Mac, and iPad. Back in 2002, Apple introduced iChat, an instant messaging service that eventually gained video calling and video conferencing support, as well as desktop sharing and more.
iMessages is proprietary and only on iOS right now. Twitter is proprietary but cross-platform. FaceTime is a collection of open standards that Apple has promised to release the specs for but hasn't as of yet. It's also iOS and Mac OS X only right now. iChat uses both proprietary (like AIM) and open (like Jabber) protocols, with different level of feature and functionality support depending on which you use (though you can use more than one). It's Mac only, but the protocols can be used in other apps on other platforms.
iMessage and FaceTime can both use the same Apple ID, and I can launch into FaceTime via a button in iMessage, but they don't really work together, not the way Skype calls and chat do, for example. Twitter is treated like a separate systems, and iChat is an island all its own.
I can't iMessage someone on Mac or Windows. I can't iChat someone on iOS unless they install a 3rd party app like BeeJive or AIM, and I can't easily switch between text video/voice in the same app.
iChat on Mac OS X could conceivably become a blended iMessage and FaceTime client. Change iChat's name to iMessage and have FaceTime calls as easy to launch from Mac OS X (and hopefully Windows one day) as they are on iOS. Add a similar way to switch back from FaceTime and things might "just work" better.
However, Apple seems to be improving the iChat app in Mac OS X Lion all on its own, with no iMessage or FaceTime support in sight. So that doesn't look like an immediate possibility.
Hopefully Apple is working on this, and we won't have to wait for iOS 7 and Mac OS X 10.8 to get a grand, unified messaging system.