Per Invisihitch, the E series X1 can tow 4400 lbs and IIRC the tongue weight is 400 or 440 lbs. You have to have a harness to handle braking on a trailer, the unbraked trailer weight is 2000lbs from what I recall reading. I'd be more comfortable towing with the 35i xDrive model (which I have :-D) or diesel.
http://www.bimmerfest.com/forums/sho...90&postcount=9