I have always had the bottom leaves on my tomato plants curl up. I usually just cut them off. Here's a link to a post with pics I had posted a ways back and you can clearly see the leaves are curled up on the bottom but also had plenty of tomatoes growing.
http://www.hydroponicsonline.com/for...-system-2.html
The following year I had the same happen and I cut all the curled leaves off and had an incredible amount of tomatoes compared to the year b4.
What type of tomato plants are you growing and how many tomatoes on each plant?