Data for home runs per game at each ballpark are now available on the Hit Tracker site.  Through Sept. 6, each MLB game had an average of 2.25 homers, with the AL at 2.32 homers per game and the NL at 2.18 per game.  Here are the top and bottom parks for homers:

Top 5 most homers per game, by ballpark:

1. U.S. Cellular Field, 3.00 hr/game
2. Citizens Bank Park, 2.97 hr/game
3. Great American Ball Park, 2.92 hr/game
4. Rogers Centre, 2.70 hr/game
5. Camden Yards, 2.66 hr/game

Bottom 5 least homers per game, by ballpark:

1. AT&T Park, 1.63 hr/game
2. Angels Stadium, 1.63 hr/game
3. Comerica Park, 1.82 hr/game
4. PNC Park, 1.86 hr/game
5. Shea Stadium, 1.92 hr/game