I found some great information about a hitch company for the f80 and a UK site that lists the weight of the trailer the M3 can safely pull. I would like to hookup a airstream basecamp for a trip across country. Looks like it is no problem to pull a 3,000lb camper.
http://www.invisihitch.com/hitches/
http://www.uktow.com/towing%20capacity.asp?make=BMW