I don't know, but mine are doing the same thing.
I thought mine might be because of the dang whiteflies that they are infested with, but now that I see this, I think we have 2 seperate problems.

See my photos under the heading "What is going on with my tomatoes" for similarity...

Do you have white flies too? I don't see any in the photo... (look under the leaves)

My tomato plants are doing something similar.. Do you live in SoCal ?

I suspect they are being caused by psyllids.. I can see lots of little bugs at the base of the plant like fruit flys.

i am trying to control them by spraying with soap.

I do live in socal, but there is no sign of any pests at all. It's not improving, I don't know what it could be..

Did you see the post ?

http://forums.gardenweb.com/forums/load/tompests/msg0622172625957.html?3

Hi -

I'm new to this site, and suspect I have a problem different from yours, but found a useful link (below).
I'm growing patio-style tomatoes in pots. In one pot the tomatoes have become somewhat stunted with stiff leaves and stems sowing yellow leaves with purplish veins. I suspect phosphate deficiency, from the photos at this other website.

Good luck!

Here is a link that might be useful: http://www.hbci.com/~wenonah/min-def/tomatoes.htm