Are tomato leafs poisons, and if so what kind?

According to these reliable sources, the leaves of the tomato ARE 
poisonous - the offending chemical is an alkyloid called Tomatine.

"Poisonous Plants of North Carolina," Dr. Alice B. Russell, Department
of Horticultural Science; Dr. James W. Hardin, Botany; Dr. Larry
Grand, Plant Pathology; and Dr. Angela Fraser, Family and Consumer
Sciences; North Carolina State University.
http://www.ces.ncsu.edu/depts/hort/consumer/poison/Lycopes.htm

Cornell University

QUESTION:  Are tomato plants poisonous?
ANSWER:  Yes. They contain an alkaloid that interferes with
cholinergic nerves and cause some serious gastrointestinal distress.

QUESTION : Which part of the tomato plant contains the Tomatine?
ANSWER :The highest concentrations are in the stems and leaves. There
are small amounts in the fruit, part of the distinctive flavor of
tomatoes and potatoes comes from these alkaloids, but at
concentrations too low to harm you.

http://www.ansci.cornell.edu/cgi-bin/db2www/plantfaq_display.d2w/report1

The poison in the leaves can be used for pest control -- 

Nightshade family plants, such as Tomato, potatoes and tobacco, have
toxic compounds called alkaloids in their leaves. These toxins are
water soluble and can be soaked from chopped leaves and made into
home-made sprays. These sprays also work by attracting natural pest
enemies. The good bugs follow
the smell of the spray in looking for prey. Tomato leaf sprays have
been used to protect plants from aphids…
http://www.tracker-outdoors.com/organic_insect_control.htm

While this spray is not poisonous to humans ON CONTACT, use care in
handling, especially if you are allergic to the nightshade family.
http://www.corecom.net/~gardener/NewsLetter/July00/July2000.html

I'm happy to have answered your question. 

--K~

Search Terms:
"tomato leaf" toxicity
"poisonous plants" tomato

A quick look for tomatine on the Hazardous Substances Database and the
Toxline database at Toxnet http://www.toxnet.nlm.nih.gov/ pulled up
the statement that it is found in higher toxic levels in the leaves
and stems of wild tomatoes, while the ones grown domestically have
lower levels. It seems that a human would have to eat quite large
quantities of leaves and stems to be affected.  On the other hand, a
large number of web sites state that tomato plants are toxic to cats.