botrytis (grey mold) can cause symptoms of damping off by girdling the stem, particularly in small plants and seedlings. it also causes infection of leaflets and petioles producing symptoms like yours. The very typical grey fuzzy appearance you will see in stock photos on mature plants and fruit only develops in wet conditions. I still believe botrytis (grey mold) to be the most likely culprit in this humid indoor situation as I mentioned earlier.
When "googleing" photos, you will only find the most typical examples on mature plants for the most part but if you are specific and google Botrytis in tomato seedlings you will find information more specific to the question at hand.
http://vegetablemdonline.ppath.corne...o_Botrytis.htm
http://msue.anr.msu.edu/news/how_to_...mato_seedlings
The situation in seedlings will be different than in mature/fruiting plants. Botrytis is common in the environment and prevention is the key as it is difficult to deal with indoors once it is there, it is unusual as a big problem outdoors unless it is very humid and wet.
fans are quite useful in indoor or greenhouse environments to help prevent fungal infections by reducing condensation and improving airflow but once a fungus is present, a fan also will help to disperse the spores and spread the infection.
I'm sorry this has happened to your seedlings, I hope you can salvage some of your work, getting them outside will be the answer if they are to be salvaged.
It will be very important to try to sanitize the area and all pots and equipment, I would recommend a 10% solution of 1 part household bleach and 9 parts hot soapy water and a good scrub of everything after your plants are out of the area.
Again, best wishes.
Karen O