I recently had a similar problem:
http://forums.tdiclub.com/showthread.php?t=494625&highlight=leak
It was quite the nuisance to repair, but another board member helped me out tremendously by providing parts difficult or expensive to obtain. The black underbody panel hiding the actual source of the leak comes off easily with eight plastic 13mm nuts that will stay in the panel. You will need to remove it to find the source of the leak. The fuel cooler itself comes off with just two 13mm nuts. Since this is a return fuel line to the tank from the engine compartment, it is not connected to any large source of fuel that will drain out. The fuel lines are a hard plastic with push-on snap-on fittings. It is not initially obvious how exactly they work, but to get them off, you push in a hard to see black button which releases two inner 'jaws' and then you can easily pull them straight off.
My guess is that you probably hit some road debris that damaged one of the fittings. Fuel will leak out wherever in the panel it finds a low spot, so the leak is probably not exactly where you think it is. If you did damage a line or fitting, the next unfortunate thing is that they are not repairable and VW only sells replacement lines as a set. To make matters worse, being able to get replacements used from an auto salvage store are also difficult, as most of these routinely use large forklifts to move the cars around, and the forks can smash the coolers and lines.
