I do not know your problem but maybe this will start you.

If the problem is a deficiency, noticing whether it started on the older or younger leaves first might narrow the cause (if they are big enough). Nitrogen & phosphorous deficiency symptoms first appearing on the older leaves. Symptoms on the younger leaves include Calcium & Sulfur. Purple leaves on the lower surfaces can be seen with a Nitrogen deficiency & leaf curl downward is seen with a Sulfur deficiency. Leaves curled under are seen with a Nitrogen excess. Slow growth can be due to Nitrogen, Phosphorous, Potassium, & Sulfur.

Here are some tomato leaf pictures:

http://www.luminet.net/~wenonah/min-def/tomatoes.htm

http://www.uga.edu/vegetable/tomato.html

Tomato diseases
http://ftsg.ifas.ufl.edu/DISMIK.HTM

Gardenweb has a neat forum on Tomato Pests & Diseases. If you think you might have a disease you could post pictures on this forum.

Any symptoms of leaf spots, abnormal veins, stem & bud character, plant size, etc., might help identify the cause.

Here are some tomato leaf pictures:

http://www.luminet.net/~wenonah/min-def/tomatoes.htm

http://www.uga.edu/vegetable/tomato.html

Tomato diseases
http://ftsg.ifas.ufl.edu/DISMIK.HTM

Gardenweb has a neat forum on Tomato Pests & Diseases. If you think you might have a disease you could post pictures on this forum.