Are iOS and Mac OS X merging? Will Mac OS XI and iOS be the same thing? Or will it happen even sooner? Only time will truly tell, but there are a number of factors which scream "yes" to me.
1. App Store
Mac now has an app store. Wow, Apple is now in a prime position to merge the Mac and iOS app stores. They both function very similarly, and I wouldn't be surprised if the only thing that separated both stores in Apple's data servers was some metadata for filtering which app would show up on which store. In other words, it'd probably be hella easy for Apple to make both stores merge. Distribution is crucial, after all. Especially since all iOS apps go through the App Store.
This one's a no-brainer. iOS apps work in full screen, and now Mac apps can work in full screen. It's a simple comparison, yes, but there could be quite a bit to it. In the initial transition, for universal apps I can easily see the iPhone variant running in windowed mode and the iPad variant running in full screen. It's a possibility.
3. New gestures
The new multitouch gestures are pretty much the same ones you can use on iOS. There is developer support which functions extremely similarly across OS's for tracking multi-touch behavior. Mac apps are also beginning to react much like iOS apps would with the gestures— very springy, with inertial controls and a "stretchy" feel are now present on the Mac-- identical to the way it behaves on iOS. If Mac users are more familiar with the same gestures used on iOS, then it will be much more natural to use iOS apps from the start. And that might be exactly what Apple is trying to do; trying to get you eased into the "iOS way" before they integrate the OS's entirely. Smooth transition!
4. Same foundation powers Mac and iOS
To put it simply, iOS runs a variant of Mac. They're both built on the same foundation, but have different "presentation" layers. All the nasty stuff down below is relatively the same. This is important, as it means that iOS apps can effectively run on Mac since they all have the same "plugs". The iOS simulator is NOT an emulator. The actual code is being run on the machine, not through an emulated abstraction layer-- that's why apps will run faster on Mac than they do on iOS. If it was an emulator, and it tried to mimic the kind of environment needed the run iOS apps, there would be massive performance hits. To put it simply, nothing is in the way of iOS code from running on Mac. "it just works".
5. Changing UI conventions
Some of the latest Mac apps have been quite different from conventional desktop apps; they've had large buttons with obvious page-based navigation. Gee, sounds like iOS! App icons now have "badges" just like iOS has. These are all small changes, but a lot of +1's really add up over time. Oh, and did I mention the twitter app? It looks identical to its iPad cousin. If the code is so portable that you can port an app from iOS to Mac with minor visual and functional changes, Apple must be doing some big stuff under the hood.
If this trend continues for 10.8, I will have very few doubts that iOS and Mac will merge. The groundwork for it has already been laid. It was possible from the start. How long has this been part of Apple's master plan? Was iOS intended to merge into Mac from the very beginning? Was Apple initially using the iPhone, back in 2007, to test if touch was the new way to go? Was the iPad an extension of these tests? I see something "revolutionary" on the horizon. And quite frankly, I'm extremely excited for what the future brings.
Don't be afraid to fact-check and correct me. There might have been some details I missed. Please let me know.