Word of OS X 10.8 (kitty code name: Mountain Lion) dropped today. The big summary is its a continuation of the iCloud-ification of your data across all devices.

Part of the announcement was a beta release of Messages.app, formerly known as iChat. Now your iMessage conversation history syncs between your OS X machine(s) and iOS devices seamlessly.

A quick test with a friend and it works as expected. "Holla" jumps up in the conversation on the iPhone, iPad and Air almost simultaneously.

So we started using it while working on our OS X machines. Then the iOS devices went to sleep. Then things got interesting.

With a conversation happening on one device but being synced across many, knowing which device is having the conversation is key to the user experience. Apple figured this out so well that I just had to jubulate in its subtlty with this post.

When typing in Messages.app, iCloud knows your in Messages.app. When typing on the iPad, iCloud knows your on the iPad. When you're engaged in a conversation on one device, the other devices do not get notified of new messages. This avoids cross-device notification flares currently experienced by calendar alerts, which makes for an especially noisey 9:55 AM for me.

Now say you're working, working, working and Messages.app is out of focus. At this point, iCloud doesn't know what you're up to. A new message comes in. Since iCloud doesn't know what you've done since the conversation stopped it notifies all your devices.

You're still in OS X so you reply to the message via Messages.app. Now you've got notifications to dismiss on your iOS devices, right? Total first-world bummer, right? Nope: iCloud dismisses the notifications across devices once the conversation picks up again.

No extra X-ing out of things. No burnt seconds of your life. Just gets rid of them because it knows you've started talking again.

This is a total win. Anyone who's had an IM client open on an iOS device while signed in on the desktop knows that the scenario of multiple notifications about the same thing at the same is annoying to the point of untenable. Power users might deal with it for its synchronous novelty, but real users won't.

I imagine the entire minagerie of Notification Center-enabled OS X apps will work this way. Maybe it already works this way on iOS and I just never noticed it (I'm stingy with what I allow in my Notification Center).

Maybe I'll finally stop being reminded of our daily scrum by three screens at the same time every morning.