Reed Cartwright has a comment on this fascinating human mutation that increases muscle mass:

(AP) Mutation Found in ‘Muscle Man’ Toddler
This is a AP report on a discovery reported recently in the NEJM.  Apparently, a German boy was born with a pair of genetic mutations that block myostatin production.  Myostatin is a negative control on muscle growth.  Thus this child who is not yet five has adult level strength.  Since I haven’t had time to grab the actual report, I assume that the actual mutations are loss of function mutation.  Instead of “blocking” the action of myostatin as report indications, these mutations probably cause myostatin production to be broken.  But, like I said I haven’t read the paper.

It sounds very cool, and I can imagine football coaches across the country drooling at the prospect of a little genetic manipulation for the next generation of NFL stars. I haven't read the NEJM paper, either, but I have read another paper on a similar mutation in cattle, a null mutation in the myostatin gene, and there may be a good reason we've got myostatin. This is a Belgian Blue with the "double muscling" phenotype:

A similar effect has been reported by the same authors in mice. Here is a truly Mighty Mouse:

Muscle in a myostatin-deficient mouse; left, facial muscles, right, forelimb. Top is a normal mouse, bottom is a mouse expressing the mutant phenotype.

Very muscular, eh? However, a note of concern for the German child, because there's more to the cow than just bigger muscles, as the paper concludes:

Unlike in mice, a myostatin null mutation in cattle causes a reduction in sizes of internal organs and only a modest increase in muscle mass (20–25% in the Belgian Blue breed as compared with 200–300% in myostatin-deficient mice). It is possible that cattle may be nearer to a maximal limit of muscle size after generations of selective breeding for large muscle mass, unlike mice, which have not been similarly selected. In this regard, even in cattle breeds that are not heavily muscled, the myostatin sequence contains two adjacent nonconservative amino acid differences (EG vs. KE) in the C-terminal region, compared with all other species examined. Although the functional significance of these differences is unknown, it is possible that these two changes represent a partial loss-of-function allele that became fixed in the population during many years of cattle breeding.

For agricultural applications, there are some disadvantages to double-muscled cattle, namely the reduction in female fertility, lower viability of offspring, and delay in sexual maturation. However, in the Belgian Blue breed, the increased muscle mass and increased feed efficiency largely offset these drawbacks. The fact that a null mutation in the myostatin gene in cattle results in animals that are still viable and fertile and produce high-quality meat demonstrates the potential value of producing an increase in muscle mass in other meat animals such as sheep, pig, chicken, turkey, and fish by disrupting myostatin function. Indeed, the high degree of sequence conservation in animals ranging from mammals to birds to fish suggests that the biological function of myostatin has been conserved widely throughout the animal kingdom.

So it might be a boon if you're raising an animal for meat, but there may be other drawbacks to worry about if you're hoping to raise a child for, we hope, other purposes. The fact that the myostatin gene is widely conserved suggests that there may be more functions to it than simply reducing muscle size than have been accounted for in the hype over this news, or that there may be a drawback to spontaneously developing huge muscle mass. Time will tell.

McPherron AC and Lee S-J (1997) Double muscling in cattle due to mutations in the myostatin gene. PNAS 94(23): 12457–12461.

McPherron AC, Lawler AM, Lee SJ (1997) Regulation of skeletal muscle mass in mice by a new TGF-β superfamily member. Nature 387(6628):83-90.