Apple developers, start your engines. Mac users, start dreaming of how much cooler your desktop or laptop experience could be this summer. That's when Apple will launch the latest big cat-themed Mac OS X, version 10.8: Mountain Lion.

At 8:30 a.m. EST Thursday, the developer preview version of Mountain Lion was live and available to Apple's legions of app makers. Mashable was briefed on the new Mac OS prior to the announcement.

Bottom line? It's a few more vital steps closer to fully connecting the experience you have on the Mac with the world of the iPad and the iPhone — dumping iChat in favor of iMessage and Twitter, to take the most radical example. It's almost as if it makes your Mac moonlight as an iPad.

But it is categorically not the one OS to rule them all, if such a thing is even on Apple's radar.

OS 10.7, or Lion, launched in July 2011; 30% of Mac users now have it installed. (Another 50% of us still favor Snow Leopard). Coming in the wake of the iPad, it was clearly influenced by the success of the device. It introduced such iOS-like features as multitouch gestures on the trackpad and a "launchpad" of apps that looked just like the iPad's home screen.

Some of us began to wonder, with some cause, whether Mac OS X and the iOS were heading for a marriage down the road.

The iOS Moves In

Well, here's the next phase in the relationship, and iOS has practically moved in to Mac's house. If Lion was a toothbrush in the bathroom, Mountain Lion is a chest of drawers in the bedroom. Reminders, iMessage, Game Center, Notifications, iCloud and Twitter integration — all iOS's most intimate stuff is here, and it all pretty much looks the same as it does on the iPad. Most of it is designed to sync up so it is exactly the same.

SEE ALSO: Apple Mountain Lion Embraces Flickr and Vimeo Sharing

And Mac OS X has had to throw out some of its stuff. Bouncing icons in the dock? Who needs them when you've got Notifications, which appear in classy banners down the side of the screen? The venerable antique Instant Message software, iChat, a 2002 vintage? A stupid wagon-wheel coffee table, says iOS. Throw it out.

Instead, here's iMessage, which will still let you IM your contacts (if you must). But what it really wants you to do is use Apple's seamless texting replacement of the same name. Admittedly, the thought of being able to immediately text anyone with an iPhone for free from your desktop is so unbelievably cool, it can bring on an attack of the vapors.

You get the sense the Mac is going to be happy with its new roommate. What's not to like about Airplay, which can seamlessly mirror your desktop on an HDTV? Or a separate Notes app, where you can attach notes to the desktop like stickies? Or Game Center, which will mean a lot more cross-device play?

Or a "share sheet," which effectively means developers are going to be able to put Twitter buttons everywhere? Mountain Lion will already let you tweet from all standard OS X apps such as Safari and Photo Booth. That means you can sit and take photos of yourself and instantly tweet them, to your heart's content. It's a boon for Twitter users (Twitter readers, not so much).

So things are going to be a lot more fun around the edges of the Mac OS — which is no bad thing. At the grand old age of 12, OS X was starting to seem a little too same-y with each iteration. This new younger partner is about to give the Mac a new lease on life. (How much that will cost, we don't know; Apple isn't announcing a price yet, or a launch date more precise than "late summer.")

But don't expect iOS to go hog-wild and bring its apps on board in future versions. Apple is giving a hefty push to the Mac app store, which benefits from a security feature called Gatekeeper where you can limit installations to just Mac store apps. (You might want to do this for your malware prone-parents, say.)

Developers are going to have to make two separate versions of apps they want on Mac and iOS for some time to come, and that's just fine with Apple. When it comes to its two operating systems, the company seems to believe living together is good enough.

So what do you think? Will you buy it? Take a quick gander at a video we put together with material from Apple, then a gallery of screenshots — and last but not least, your chance to chat up a storm about this major Mac development in the comments.

Apple Unveils Mountain Lion

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