Most recent article I could find in a google search was from 2008, but....
It has revealed that 1,563 games were given ratings last year, a 22 percent increase from 2006. The E for Everyone category continued to have the most games, with 59 percent of ratings, followed by T for Teen (13 and over) with 20 percent, and E10+ (10 and older) at 15 percent. M for Mature (17 and older) rated games made up 6 percent of the total, down from 8 percent in 2006, and 12 percent in 2005.
A statement from the ESRB president, Patricia Vance, said, "Although we tend to hear from the media most about the more controversial Mature-rated games, the reality is the 8.5 out of every 10 games sold last year were rated as appropriate for ages 13 or younger."
M rated games:
2005 - 12%
2006 - 8%
2007 - 6%
Even if there's been an upswing, it's still doubtful that it's even 15-20%.
But like the article said - they get the press. (Also, as a fan of M rated games, you pay more attention to them.)
Personally, I have no problem with T-rated games. But that's because I like all sorts of different genres of games. SSX snowboarding games, Ratchet & Clank platform games, puzzle games, Wii golf & bowling games, etc. "M" isn't some great special thing that suddenly makes a game better, any more than "R" is as a movie rating. :shrug:
(huh, just checked - I'd forgotten that the SSX and R&C games are actually rated E, not even T. )