Universal Binary will support a new processor architecture after Apple (or someone) develops the compiler for it. I believe Apple is using the GNU compiler for Core Duo. So, there is always work involved when supporting a new microprocessor. Speaking of GNU, it is amazing how we now live in a world where companies can deliver innovative products based on open-source technology. Ten years ago we couldn't have had this discussion.
Apple made a smart move migrating to Core Duo, because they needed something new. I was surprised they didn't migrate to AMD, because Opteron is so good and is 64 bit. Core Duo is a better choice for notebook computers (a very important market segment.) The migration was also well timed from a marketing perspective. Apple has held on to public mind share because of constant product introduction (mostly iPods) the past couple of years. One of Steve Jobs' strengths is his initiative. Apple is proof that a smaller company with initiative can outperform larger competitors. Larger companies tend to be very risk averse and also slow to react due to the bureaucratic nature of their decision making processes. I believe Sony is in this category and is fighting hard to change.
I believe QT does abstract away some differences. It actually doesn't have to, it is just a matter of how much effort goes into developing QT. Some software companies have the resources to develop their own custom QT widgets. I agree that developing to Carbon/Cocoa is the way to go if you are writing only for the Mac.