Hmm. I've honestly never written any code that requires intimate knowledge of what the kernel is doing, let alone modifying that behavior; are you certain you can't run your program in userland?

If you've got a command-line C program, you really shouldn't need to do any adaptation at all -- both the N900 and the N9 are running Linux, after all. The only significant difference between the two, for most apps anyway, is the user interface. If you've got a dependency on specialized libraries, that might be a problem, if they haven't been ported to the N9 yet. (And if you really really do have a dependency on a customized kernel, well, I just don't know what you are trying to build...)

Certainly, Qt-based code must be ported from one device to another; we are talking C++ here, not Java. But the big problem with Symbian is that only a relatively small portion of Qt is supported on it; I think that's the main reason why they push so hard to get Symbian folks to use Qt Quick instead.

For many apps, going from, say, Windows to Linux becomes a heck of a lot easier to do if you don't have to rewrite the user interface. Implementing your UI in Qt makes life much, much easier.