This is really a biology (human genetics) question, not a philosophy question, but I suppose that there is no better fit among the Stack Exchange sites.
I also need to clarify a few of the points of biology.
First, the question is mostly about selection. Although there are random processes that affect biological evolution, only selection turns evolution into an optimization algorithm. Hence, it does make sense to consider what kind of selective pressures exist, and consequently, what is being optimized. Thus, the question is best phrased in terms of selective pressures affecting human evolution.
Second, let's keep in mind what "fitness" actually means with "survival of the fittest". You could be able to run a marathon, bench press 300 lbs, and so on, but if you have no children (and do not aid siblings of yours in a way that allows them to have more children), your fitness is zero. Fitness means only "reproductive fitness", though to be fair it should be considered arbitrarily far into the future (e.g. if you have 20 children none of which will themselves have any children, your fitness should again be considered zero (at that point)).
So what actually matters to fitness? Let's leave aside the more complicated (but important!) case of assisting relatives, and focus just on the individual aspect. The two things that matter are: (1) How many children do you have? and (2) Do those children survive until they're old enough to have children themselves (and, in fact, do they)?
It is likely true that for a good portion of the world's population, physical fitness and reproductive fitness are being decoupled, because we have many technological alternatives to intrinsic strength and robust immune systems (cars, antibiotics, etc.). Also, for a smaller portion of the world's population (i.e. China), the number of children is also approximately fixed.
So for China, the answer to the question "what selective pressures are there" is "essentially none" (except for essential biological processes necessary to even survive until birth). In other industrialized countries, the selective pressures are in favor of people who have large families; most everyone survives, so only the birth rate remains. Much of the "developing" world has not seen a dramatic change in selective pressures, though details of availability of food have changed somewhat.
The consequence of a lack of selection is an increase in the diversity of the gene pool, much of which would have been maladaptive in earlier more stringent selection environments. Genetic defects accumulate slowly, so one doesn't have to worry much about drift until hundreds or thousands of generations have passed (and I would be very surprised if the same conditions persist in e.g. China for that long).
The consequence of selection for large families is more interesting, and could change the gene pool in only a handful of generations. For example, if there were a (dominant) gene that caused those who have it to have a family twice as large as average, after only 10 generations, it would be a thousand-fold overrepresented compared to the small-family variant. Of course, the earth can't tolerate a 1000fold increase in population, so eventually this alelle may become disadvantageous (as everyone in the family suffers to such an extent because of the large size that even though there are more of them, they are less reproductively fit than a smaller family). Still, it doesn't matter why the gene was effective; if it made someone really love children, or gave them a latex and contraceptive allergy, or predisposed them to Mormonism (or another religion that promoted large families), it would have the same impact: that gene would be favored. The question, though, is whether such single genes exist. Because, of course, if it takes many genes to generate a large difference in family size, almost none of the children in that family will actually have all those genes. So the process of "fixation" (i.e. those genes taking over) will be slow. In most cases, although genes do appear to play an important role in many human behaviors, the number of genes is large. Thus it is likely (although here we can only speculate, whereas in the genetic drift case we know) that it will be many generations before this would have much impact either.
(And, of course, don't forget the large fraction of the world's population where the pressures are different.)
So, bottom line is: in some places yes, we're approximately at a plateau but plateaus are not so bad in the short term; in other places no, there are sizable pressures still; and we probably have quite some generations before we'll see major consequences from those pressures.
Eventually we as a species will have to deal with these issues. Presently, however, it is not one of our most pressing concerns. (Let's get resource depletion, overpopulation, global warming, etc., all dealt with first.)
