What does this mean for self-control as a virtue?

While the test doesn't prove that the virtue of self-control isn't useful in life, it is a nice trait to have; it does show that there is more at play than researchers previously thought.

The key finding of the study is that the ability of the children to delay gratification didn't put them at an advantage over their peers from with similar backgrounds. Students whose mothers had college degrees were all doing similarly well 11 years after they decided whether to eat the first marshmallow. The same was true for children whose mothers lacked a college education.

This opens the doors to other explanations for why children who turn out worse later might not wait for that second marshmallow.

Many thinkers, such as, Sendhil Mullainathan and Eldar Shafir, are now turning to the idea that the effects of living in poverty can lead to the tendency to set short-term goals, which would help explain why a child might not wait for the second marshmallow. If true, then this tendency may give way to lots of problems for at-risk children. After all, if your life experiences tell you that you have no assurances that there will be another marshmallow tomorrow, why wouldn't you eat the one in front of you right now?

They often point to another variation of the experiment which explored how kids reacted when an adult lied to them about the availability of an item. When a child was told they could have a second marshmallow by an adult who had just lied to them, all but one of them ate the first one. In the cases where the adult had come through for them before, most of the kids were able to wait for the second marshmallow.

Were the kids who ate the first marshmallow in the first study bad at self-control or just acting rationally given their life experiences? The same question might be asked for the kids in the newer study.

What does this mean for experimental psychology? 

The refutation of the findings of the original study is part of a more significant problem in experimental psychology where the results of old experiments can't be replicated. Some tests had a poor methodology, like the Stanford prison experiment, some didn't factor for all of their variables, and others relied on atypical test subjects and were shocked to find their findings didn't apply to the population at large, like the marshmallow test.

That last issue is so prevalent that the favored guinea pigs of psychology departments, Western, educated, industrialized, rich, democratic students, have gained the acronym WEIRD. This is a bigger problem than you might think because lots of ideas in psychology are based around the findings of studies which might not be generalizable. The original marshmallow test has been quoted endlessly and used in arguments for the value of character in determining life outcomes despite only having students at a pre-school on Stanford's campus involved, hardly a typical group of kids.

The Stanford marshmallow test is a famous, flawed, experiment. While it remains true that self-control is a good thing, the amount you have at age four is largely irrelevant to how you turn out. So, relax if your kindergartener is a bit impulsive. They still have plenty of time to learn self-control.