Last year, 2009, MLB showed a significant drop in attendance, compared with 2008. The biggest one year drop since Harry Truman was President, according to the USA Today headline. Some were of the opinion this was a one year occurrence which was due to the economic troubles, and that MLB would quickly rebound in 2010.
When spring training attendance showed an increase in 2010, some believed that this indicated MLB's attendance problems were over, that regular season attendance would increase in 2010, and there was nothing structurally wrong with the game which could be causing the attendance problems. The worst economic troubles were in 2009, and NFL attendance didn't show a big decrease, so the thinking was that MLB attendance would rebound in 2010.
So far, this has not been the case. MLB attendance is down by half a million compared with 2009. The season is now more than half over, and it is starting to look like this will be the second straight year of decreased attendance for MLB. Although the decrease won't be as dramatic as last year, this trend must be worrisome to MLB.
Here is a link to site which provides daily updates to 2010 VS 2009 MLB attendance.