Wow, lots of activity today.

Want to thank everyone who's spoken up so far. Want to also thank Kobu for his pre-emptive proof of concept.

To expand on my previous post, I think you should actually lose Karma if you demand payment for stuff while you have Good Karma (in other words, if you've got good karma it should move you towards having Neutral karma, while if you've go bad Karma it shouldn't have any karmic effect upon you).
This is something I've definitely considered, especially as part of the larger dependent dialogue/game-play mod.
Basically, what I'm going to have to end up doing is deciding on a baseline "morality code."
I am NOT certain at this point just what that will be, but it should be generally common sense and will likely be based on general concepts, such as Altruism and Honor, as opposed to societal norms. An example of an area of contention would be the theft of goods from dead people: in our society, grave-robbing is generally considered wrong, but if you grew up in a harsh wasteland, the idea of taking supplies off dead people may not be considered particularly evil as a whole (although it may strongly irritate people who see you do it and know the victim). The game doesn't really approach this, since you can loot bodies -- taking even the possessions of dead friendlies right in front of their family/friends -- and with the addition of graves in FONV, you can even DIG UP dead people to steal things without any karma loss. If, however, I decided to base the Moral Code on what I think would be more appropriate in post-apocalyptica, then to balance it out I'd have to make other things (like murdering people) hit harder: I don't want to ENCOURAGE just killing people to steal their stuff without the karma loss.
Something I wanted to come away from was gaining karma by murdering Bad Guys, or losing it by defending against "Good Guys". This is silly. Without the capability to "show mercy" in the game, successfully surrender, or disengage from combat (short of running away), there is no viable reason that you should gain or lose karma for self-defense. This shows with the feral ghouls, but it also means that if a pack of Rangers come flying at you from over a hill, only the most pompous, judgmental, self-righteous ***** in the world is going to blame you for shooting back. On the other side of the coin, I found it kind of silly that I amassed a legendary amount of Good Karma by sitting on a hill with a scoped varmint rifle and taking out almost every bad guy I could reach -- assuming I even KNEW they were bad guys from a mile away.
In the end, karma should reflect the deliberate choices you've made that are for or against the baseline moral code. I think this is BEST done by how you interact in dialogue, how you resolve quests, and how you behave pre-emptively (i.e, killing villagers outright, vs. killing them in self-defense).