It is highly unlikely Apple will merge the two OSes within the next foreseeable future. Give it 10 to 20 years and that may happen, but even then, so many things would have changed everything you know now will be considered dated - just like creating shell scripts in Perl is considered dated.

The problem is the two operating systems are designed for different purposes, extremely different purposes. One is designed for mobile devices, to receive primarily touch driven events, and works off an extremely small hardware base. OS X, Apple's laptop and desktop operating system, is designed for many more, much more intensive things - relying a lot more on things you would not find on a mobile device, such as file management, users, permissions, terminals, third-party installable applications, process management, and all the things that come with Apple's graphics engines that so many people seem to like.

Overall, the chance to create new desktop software that will survive the test of time is limited. Within the next few years it's far more likely you will see intensive desktop applications go to simpler and easier to use web applications. Starting a new desktop app currently may prove to be a waste of time, depending on what it is.

Mobile apps will be around easily for the next 5 to 15 years, as the general population will still be growing the percentage of people who have smartphones, who use mobile apps, and therefore the demand for new mobile apps will increase.

Personally, if I had the choice, I'd work on mobile apps more. I myself do a lot of Android development, trying to avoid developing specifically for iOS devices. You may consider learning Android SDK to grow your horizons, or even the new BlackBerry one - which I read somewhere is offering a guaranteed $10,000 for any Blackberry-specific new app created for the first year. Obviously 10k in one year isn't talking about a huge jackpot, but that may get you interested to know that at MINIMUM you will earn 10k from Blackberry's application marketplace next year.

However, at the same time, I work with web applications more than I do mobile applications, and the only growing demand I see for desktop applications are in integrating web services more deeply with an operating system and video games (which honestly, never stops growing the demand for). So if your desktop app is one of those, go with with that hands down.

If you're looking at taking on a paying job developing a desktop application for Mac OS X versus working for yourself on your own mobile applications - work for yourself as long as you can. Trust me, I used to have a few semi-popular Android apps, made enough to keep a stable lively hood, then took a job to get a little extra money, within about two months of not having the time to commit my mobile apps dropped, I went from a peak of 15k a month to now roughly 800 a month in mobile app income - which removes my option of working for myself until I stabilize that again. It is incredibly difficult to do so however, when balancing a full time job and having the desire to work for yourself again. I now know never to make that mistake again. Career wise I would have been far happier earning a little less but working for myself, on my own schedule, than ever working a 9-5 job.