Bear, good suggestion.
If no cracks are apparent, also be sure to check on top of the left side of the fuel tank where the fuel line connects to the tank. This area is notorious for rusting out where the metal hose fitting is connected to the tank, as there is a slight depression that collects water and corrodes enough to form a hole over time.
When the tank is full, the fuel will well up in to the depression. In fact, when my E30 was full and inadvertantly parked on a slight slope with the left side downhill, the fuel would actually puddle on the ground. In my case I had to
replace the fuel tank since the corrosion was so bad. (northeast car)
Hopefully you don't have the probelm this severe, as you will find the replacement procedure
http://greg.scott.com/bmw/gastank.asp to be a barrel of fun.