The 5x M9 salts recipe here seems to be almost identical to the 10x M9 salts protocol we use in our lab and which I also found elsewhere (see e.g. http://www.changbioscience.com/protocols/recipe/M9salts10X.htm and http://www.atcc.org/mediapdfs/2057.pdf). It could be that bacteria don't mind if they get the double of salts, but it might nevertheless be interesting to know what conc. of salts would be best to use... (MDolinar)
Miller Vs. Sambrook
At first I thought that this discrepancy may have arisen due to a miscalculation somewhere down the line brought about by the difference in molecular mass between Na2HPO47H2O and Na2HPO4, but I did a little digging and it seems that the real difference is who you trust more: Sambrook or Miller. The [Sambrook protocol] for 5x M9 salts uses 64g Na2HPO47H2O while the [Miller protocol] for 5x calls for 30g Na2HPO4 (hence 60g Na2HPO4 in most 10x recipes).
In the end, the two recipes are only slightly different when you take the difference in hydration into account. When you make 1x medium from 10x Miller M9, you end up with ~42mM Na2HPO4, while 1x medium made from 5x Sambrook M9 leaves you with ~48mM Na2HPO4. Presumably, this is close enough.
J. Sambrook, D.W. Russell, Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory Press, New York, ed. 3, 2001) pg. A2.2
J. Miller, Experiments in Molecular Genetics (Cold Spring Harbor Laboratory Press, New York, 1974) pg. 431