According to WNST in Baltimore, the Bengals will open the season in Baltimore on Monday Night Football on Monday, September 10th. Take it for a grain of salt for not as this is not yet verified until the league releases this year's schedule at 7 PM ET.
UPDATE: Both Geoff Hobson of bengals.com and Paul Dehner, Jr. of CBSSports.com are reporting the same news. We'll post more after official word comes out with the NFL's schedule release event in just over an hour.
During the 2007 season, the Bengals hosted the first Monday Night Football game of the season. They beat the Ravens in Cincinnati 27-20. If the Ravens do host Cincinnati on Monday Night Football, it would be the first Monday night game in Baltimore since December 3, 2007 against New England.