There are two more reasons (see previous post) why leaves droop when the sun comes out and that is because of fusarium and verticillium wilt. Both of these are fungal infections which are found in soil and infect the roots and clog-up the vascular system of a tomato plant preventing it from carrying moisture to its leaves etc. a bit like a human with very slow blood circulation (excuse the comparison!).
There isn't much that can be done apart from foliar spraying to help the plant in hot weather but an infection of either of these wilts will slow down the production capabilities of tomato plants depending on the severity of the infection.
Once again, use fresh, sterilised compost from grow bags etc. to avoid this problem. It usually occurs when plants use the same soil several seasons in succession.
You may have noticed that there is now Twitter on the blog. Twitter is a sort of mini blog where I can jot down a word or two about what is happening in my garden with regards to my tomato plants at various times of the day. I'll give it a try and see what happens!