John, a google search brought up a possible solution to your problem.
http://photo.net/digital-darkroom-forum/00QSap
It seems other were having the same problem you were with the DNG converter not liking the files from the XSi. According to the forum, make sure you have the newest DNG converter and have your Camera RAW updated. I think the individual in the forum reinstalled their Camera RAW update to solve the problem.