Um, I'm not sure I understand correctly. KDE is just another Linux application (I'm not sure, but I suppose something similar to cocoa). And Konqueror is the old file/web browser for KDE. This has nothing to do with the ability to execute a Mac application under Linux. And on that note, KDE also runs on many BSD systems as well.
Now the question of running a mac app in linux; as many have said, simple answer: no. You have to modify most applications so they would compile under Linux. This is a harder feat when going from mac to linux b/c most of the mac software aren't open source. So even if people want to, they can't. Going the other way however, there are things like Fink. Fink is by no means perfect, but it's better than nothing.
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You can always do what the crossover people are doing, implement one OS's api into another. IE, windows API into Linux and now, Mac. You'd just have to implement the Mac OS API into linux. I don't know if kernel modification is necessary though...