Wow - the level of intelligence is reaching critical mass... I guess it balances out in the sister thread to this debate.
The idea of objective morality was best put earlier by RNB as pleasure = good vs pain = bad (and I apologize - my metaphysics was never that strong and so I see this primarily in a biological context as before). The basic selfish biological drives can guide behaviour whether both consciously and subconsciously across most social or interacting species. Simplistically, it's the same sort of learning children go through when they learn not to touch hot. Hot brings pain, don't do it again.
The idea of stealing becomes far more complex because we have very specific definitions around the act. But if define it as cheating or lying, you then find context for it outside of humanity. Cheaters are those who try to either dupe fellows into aiding them and then not reciprocating, or to take more than their share by distracting others. Naturally, much like the thief who takes your money, there is big short term gains. But in the long term, the cheaters are identified and their success drops and their fellows will even shun them. (Generalization of Evolutionary Game Theory which basically demonstrates that cooperators come out ahead in the long run - Wiki actually does a decent job of the Tit for Tat reasoning: http://en.wikipedia.org/wiki/Tit_for_tat
How does that apply to society? Well, even though I would agree that most morals now have subjective definitions (and some are purely so based on culture - don't eat beef? Crazy.), the basis comes back to the simple pain v. pleasure or if you will the Golden Rule. Stealing has been unanimously set up as amoral because it causes suffering to one end. As for the stealer, even if there were no punishment, trust and membership to the society would be inevitably cut by individuals (assuming he was caught, of course) who would fear a similar suffering if they were to trust the stealer, which would mean short term gain and long term loss for the thief. Instincitvely, it makes sense. Many other of societies norms will fall into a similar pattern. Humans have just added complexity to the simple instinct - language, customs, social strata, legal systems/deterrents etc. that muddies the waters a little when talking about morality.