I've been growing grafted tomatoes for a few years with no problems, so this is a new thing (in my experience, anyway). I should also add that my non-grafted tomatoes are fine. This is also happening to most of my mother's grafted tomatoes up north, so it's not an isolated problem.
This is what happens: they start off growing fine, then the leaves begin to curl and form a tube lengthwise down the leaf. They are still fully hydrated and green, just curled up. They flower as usual and the tomatoes begin to form, but they fail to develop very far. They eventually go brown. The final stage seems to be the stems going brown and the plant beginning to die. By this point it will have spread to any nearby grafted tomatoes.
Mum says she's seen a similar thing happen to tomatoes in her area (which has a humid climate making things prone to fungal infections etc) at the end of the growing season, but not this early. It's very interesting that it only seems to be happening to the grafted ones: I know little about these things, but since there's only a couple of companies producing grafted tomatoes in NZ, I'm wondering if someone got careless at the grafting stage and introduced an infection into the grafts? Either that or our sunless, damp "summer" is taking its toll, and the grafted ones are just more vulnerable? If anyone else has had this problem I'd be interested to hear from you. Thanks!