Have you checked with a professional to see if its ok to vent the dryer back into the house? If you have ever looked at the outside vent, it has to be cleaned regularly because there is lint that doesnt get caught in the inside lint trap. You could be filtering air contaminants back into your house, and perhaps issues with carbon dioxide or something. Its also a fire hazzard. I'm no expert, but i would assume theres a reason for an outside vent to begin with, probably a safety reason!
edited: I just looked online, there are various sources that say you must vent the dryer outside! the moisture causes mold growth in the home, air contaminants, and the loose lint is a huge fire hazzard!
Edited 11/5/2009 8:08 am ET by amomalmost40