Nezumi, good point.
I think that karma represents, for most uses (ie, rerolls and other luck related things aside

) the
effort that a character puts in to learning something. The more karma you earn, the more work you've done. Challenging things reward you with more karma. You then "spend" that karma to learn a skill -- signifyin NOT that you suddenly get discretely better at {shooting/jumping/intimidating}, but rather that you have been working on it, and that now you are allowed to reap those benefits in-game.
Like the page that Backgammon linked, I think it's reasonable that average joes would be able to gain karma/ learn skills related to things they DO. If they put effort in to learning a martial art, or regularly go to the gun range, then sure -- I'd expect them to get better. Admittedly, it doesn't follow the way the BBB says that player characters gain skills/attributes, but I think it still makes sense. Similarly, I think that it would be reasonable for a GM to reward a person with skill in something if they practice at it, work at it, etc.
For example, character X hijacks a tank and drives it off into the jungle, delivering it to some local rebels he's helping. His GM might reward him directly and say "You now have Tank Driving 1", or conversely, he might award him a little extra karma, but give the stipulation that it cannot be used to raise anything BUT that skill.
Yeah, I know, kindof radical, when you apply it to other skills, or skills the character already has. But, imaine this -- your semi-munchy Gunbunny decides to help with negotiations. You know that they are saving every bit of karma to upgrade that SMGs skill to 7, but you'd rather see them get more balanced - so you can award them this extra karma, with the stipulation that they spend it on negotiations.
Hmm... that smacks of railroading when I think of it that way. Maybe it's not so good after all, lol. I think the core idea is sound, but that restraint would have to be used when applying it.