If that happens (and it almost definitely won't, at least for a long time), then switch. Making a decision based on pure speculation is a terrible idea. There are plenty of good reasons to switch to Ubuntu, but "I have a feeling Apple has a secret plan to turn OS X into iOS" isn't one of them.
Yes it is. They make much more money off of iOS than they do Mac OS. Therefore, Mac OS will become iOS because Apple isn't your friend they are trying to make money just like any business.
Ford makes more money selling trucks than they do selling anything else, but no one would argue that therefore all their cars will become trucks because Ford isn't our friend.
They are two products serving two populations, and it seems unlikely that their timelines will converge anytime soon. However, "not converging" doesn't translate to not sharing. There are obviously some things iOS that would be at the least nice to have in Mac OS (the app store being the one of discussion now), just as there are things in Mac OS that would be (or already have been scooped and are already) nice to have in iOS.
@jpdbaugh, it's not an either-or proposition, it's a both-and proposition. Creating an app store will make the Mac both better for "average" consumers AND creative developers who want to monetize. Additionally I think increased usability from increased touch gestures and advanced expose functionality will help everyone. It's very difficult to argue that OS X advancements have been tailored to strictly one the consumer or the other creative professional: Time Machine, Spaces, Expose...these have been a win for everybody.
I think if there's something to learn from Apple it is trying to take seemingly divergent markets or divergent goals and finding creative ways to reconcile them: form and function (look at the battery compartments in their wireless keyboard, for example), cost and sustainability, business and consumer markets, young and old.
Despite the fact that unifying two codebases makes much more sense than selling a truck to a family I must say that I disagree.
The problem for me is not convergence per see, I would not mind having features from an portable OS in the OS that I use in my laptop or workstation, the problem is the impression that since iOS devices came to exist OS X is becoming more irrelevant, I think this tweet summarizes the situation: http://twitter.com/iTod/status/27297928138#
If the iOS model of a locked down consumption oriented operating proves to be more profitable that is what Apple will focus on. Its not evil or wrong it will just be what makes the most money. Its really that simple. Apple isn't out to make the core group of users happer but rather the casual user.
