Humans are mammals (but it's been painfully long since I've been offered proof of that, and don't even think of saying anything clever about that comment), and as best I know, there are two basic strategies that mammals (OK, and other vertabrates, but I had to get that bit in there) pursue with regards to population growth. There are r-strategists, which tend to reproduce very quickly and exponentially, and are adapted to very unstable environments, and k-strategists, which prefer stable environments and breed very slowly to keep their population at or just under carrying capacity.
I tend to think that humans are K strategists. While no organism is purely a k or r strategist, I think out long wait until sexual maturity, prolonged childhood and relatively low reproductive rate (compared to say, a rat) means that human populations will taper off at some point. Call me panglossian and my glasses rose tinted, but I really don't think people will breed themselves into starvation. Nature, in her infinite wisdom, has struck a careful balance between mens' enjoyment of sex and food.
Actually, that's not the reason I think a population crisis isn't looming. Men obviously enjoy sex more than food. There is, however, a drop in fertility when nutrition is poor, so there are at least some barriers as to how bad things can get. Hopefully other factors will rescue us from our own fecundity first, however.
The first is that as the world increases in general prosperity and medical sophistication, which I actually believe it is (though sometimes when I read about the popularity of medical nonsense I question this), I think the average number of children per household will go down. In the roughest sense, increasing prosperity tends to result in smaller families. There are obviously many more factors than that, but when you have (as I am blessed to, living Stateside) the assurance of having most of your children live past five, you really don't have to be churning out so many to ensure progeny in the next generation. In a very loose sense, better medicine changes humans from r strategists into k strategists.
The second, slightly less optimistic, reason I think the human population will never exceed carrying capacity enough to threaten happiness on average, is war. War, contrary to popular notions, doesn't actually kill that many people. Getting lots of people in the same place does tend to spread disease, and in fact WWII was the first war with more casualties from enemy action than disease, but historically the actual percentage of populations killed in wars was rather low.
This is begining to change. I forsee war becoming more like it was in the Middle Ages, except with more people. Most countries won't have very well trained or organized national armed forces, and will end up relying on mercenaries or similar means to fight their enemies. Infantry may well become more important, because specialized machines of war are just getting too freaking expensive. There's a reason that they can only buy 339 f/a-22's, or whatever neutered remenent of US air superiority Congress is going to authorize to protect my skies for the next twenty years. It's because to remain competative, a war machine has to be incredibly complex and expensive.
Therefore, if you're not a super power, you will soon only fight wars when you really really have to. Furthermore, holding to silly rules, treaties and restrictions won't be an option. Got chemical weapons? Good, you're going to need them because your enemy had biological ones. So far humanity has show, well, inhuman restraint in not deploying chemical and biological weapons (which are far more dangerous than nuclear ones because they're cheaper), but as wars get increasingly more vicious, expect the gloves to come off.
The increased casualties, then, will come in the form of civilians. Taking WWII as the prototype for future strategic war planning, the majority of people killed in future strategic (and they'll all be strategic since the frivolous ones won't be affordable) wars, it's always more effective to destroy industry and national morale than to shoot at tanks. This doesn't mean less casualties, however, by more efficient I mean it takes fewer bombs. You can get a lot of civilians with one bomb, the way they bunch up (and will do so increasingly as population increases).
Future wars, then will kill large percentages of the populations in areas where resources are in dispute. It'll be another constraint on population growth, humanity's inability to put up with large amounts of itself. It may result in lots of ugly wars, but it may well prevent more casulties of want and starvation.
The third factor I see keeping humans from breeding themselves into starvation is increased acceptance of birth control. My understanding is that better education (which I see as a worldwide trend) results in more birth control. With the spread of AIDS worldwide, condom education has become increasingly important (at least where the leader of the country isn't a complete idiot, interpret that statement as you wish) at keeping the population from getting completely wiped out by that particularly virulent disease. One can certainly hope that family planning will piggy-back on general sex-ed and keep us all from having to kill each other to make sure there's enough food to go around.
