This is purely a guess from the people I know and know of, but it seems to me that the most common middle names (in the U.S.) are those which begin with M. I first thought of this after tregoweth pointed out that in women near his and my age (born early 1970s) "Michelle" or "Marie" was a good guess for anyone's middle name. Upon thinking about it, I thought of all the guys I've known whose middle name is "Michael" (plus one "Manuel"). And then I thought of the book Harriet the Spy, in which occurs the following scene:
"Children, this is my mother, Mrs. Golly. Mother--you can close the door now, Mother. This is Harriet Welsch."

"Harriet M. Welsch," Harriet corrected.

"You know perfectly well you have no middle name, but if you insist, Harriet M. Welsch."

So something about the initial M makes it very popular on middle names. Perhaps the Marie, Michelle, and Michael are a borrowing from French culture, where hyphenated names such as "Jean-Michel" or "Anne-Marie" are fairly popular, but things like "Jean-Claude" that don't have an M are just as common. I think "M." sounds good as a middle initial (of course, being as how my middle name is Marie, that may just be from my habit of signing myself Suzanne M. Saunders) but not necessarily better than any other letter.

However, icicle's comment in the (now-deleted) my middle name writeup that her mother believes in middle names having the accent on the second syllable also makes sense applied to the middle names I've mentioned.