The fuzzy hubby and I are currently working on a FAQ for using a MINI as a towd (dinghy).
Short answer is that you can flat tow with no modifications if you have a manual transmission but it will void your warrenty. You may flat tow an automatic transmission with modifications costing around $1000, available from Blue Ox. Or you may tow either with a tow dolly and keep your warrenty.
Here's some pictures of various towing setups: