Forget the suspension next season, if you're trading for Big Ben you're trading for a guy to be your franchise QB for the next eight years, plus four games, a team can survive and make the playoffs as long as they take one or two of those games. The biggest question teams have to ask is would you rather take a chance and give 50 million plus to a 22 year old QB with a top ten pick like Bradford, Sanchez, David Carr, Harrington, Leftwich, Stafford, etc... when there is less than a 50/50 chance they'll develop into a Pro-Bowler. Akili Smith, Cade McNown, Tim Couch, Vince Young.
I think the reason why no one is jumping to get Roethlisberger is because the level at QB has never been higher than it's is now. Brady, Peyton Manning, Eli Manning, Romo, Aaron Rodgers, Matt Ryan, Joe Flacco, Carson Palmer, Phillip Rivers, Matt Schaub are all A-list caliber QB's, and there's a solid list of B caliber guys Matt Cassel, Chad Henne, Jay Cutler. If this was 2000, teams would have made the move already for Big Ben, but this day and age, teams believe they can develop a QB or pick up via trade or free agency and still win a championship.
Basically the Steelers if they want to get rid of Roethlisberger will have to settle for a couple 2nd round picks like the Broncos did with Brandon Marshall, then maybe a team like the Broncos or the Rams would pull the trigger.