Another bit worth mentioning from Siricusa’s Mountain Lion Review regarding how Apple handles (or doesn’t handle) messaging in the new Messages applicaiton (it replaced iChat):

The new interface combined with the new protocol leads to an experience that I found confusing. Send a message using the Messages application and a window may appear on one or more Mac screens, a notification dialog (and sound) may appear on iPads or iPod touches, and someone’s phone may vibrate in their pocket. Oops, did you just mean to send a message to your colleague down the hall as he sat at his Mac? Or did you mean to send a text to someone’s phone and not cause an alert to appear on his iPad which is currently being used by his child to play a game?

This is going to be a tough nut for Apple (and everyone else making mobile devices) to crack. Many of us are living in a multi iOS device world and can benefit greatly when all our devices are data- media- and message-synced. But only once we have said synchronization do we realize all the rules needed to make such a feature useful.
The same thing happened with the photostream in iCloud. When I first set it up, I thought, “Great! Now I have all my photos on all my devices.” I was only after living with photo synchronization did I realize I didn’t want every photo synced on every iOS device I own. Eventually, Apple added the ability to remove photos from Photostream on a per-device basis.
I see the same thing happening here, it’s all about Apple’s baby steps. Get to core functionality down, then slowing layer in added controls and customizations. In this case, mirror messaging across all devices on a particular iTunes account and once messaging is working give people the ability to control what devices get what messages and when.
Remember, when Apple introduces a feature, they want to get it right the first time, like copy-and-paste. I’m willing to bet they’ve already been working on mutli-device message synchronization. It’s just not ready for prime time yet.