I saw this link to a post on a Broncos blog talking about how their homefield advantage is really suffering lately.  Who can forget them blowing n fact, it pretty much shocked me how bad they've been.
http://broncotalk.net/2007/09/wheres-the-magic-in-mile-high/

We all remember their chokejob against the 49ers that kept them home last year but in their last seven home games they are 2-5 and are a botched snap on an extra point and a Janikowski field goal away from riding a seven game losing streak.

The author of the post thinks it's a combination of the new stadium and the fans not getting as into the games.  I hope that Arrowhead can stay as loud and as good of a homefield advantage as it has traditionally been with all of the new additions.  I think it can as long as the fans keep it that way.  Unfortunately it happens everywhere that when ticket prices go up so do the number of fans that just want to sit on their seats and watch the game instead of standing up and affecting the game.