From Michael K (07.06):

Burnt toast is high in benzoapyrene, aka benzopyrene, a five-ring polycyclic aromatic mutagenic hydrocarbon that is highly carcinogenic, because it can cause DNA mutations. It is a crystalline yellow solid. Benzo[a]pyrene is a product of incomplete combustion at temperatures between 300 and 600°C. It's probably not enough to worry about, though.