Here is what it says on Dr. Greene's site
The concern is with infant botulism.
Botulinum spores are found widely in soil, dust, and honey. Adults who swallow botulinum spores are almost never affected. When infants swallow the spores, however, the spores can germinate in their immature gastrointestinal tracts and begin producing botulinum toxin. This has occurred even when the honey was only used to sweeten a pacifier (European Journal of Epidemiology, Nov 1993).
Botulinum toxin is the most poisonous natural substance known to man. The lethal dose is only 0.0000001 mg per kg of body weight -- an amount that would be invisible to the naked eye. This tiny amount in the blood stream can cause death within minutes through paralysis of the muscles used in breathing.
Infant botulism has been found on every continent except Africa (Journal of Perinatology (2007) 27, 175–180). In the United States it is most common in the states of California, Utah, and Pennsylvania. While infant botulism can occur from taking in soil or dust (especially vacuum cleaner-bag dust), eating honey is the number one preventable cause. Corn syrups are not sterilized and may also be a source of contamination (The AAP Red Book, 2000).
Infant botulism can occur any time in the first year of life, but like SIDS it is most common in the first six months. In fact it has been suggested that it might be the cause of death in up to 10% of SIDS cases (Nelson Textbook of Pediatrics; Saunders 1992).
Thankfully, in most instances of infant botulism, the amount of toxin is so incredibly minuscule that the case remains mild. For this reason it is often misdiagnosed.
The first symptom of infant botulism is constipation (which is also a common benign finding in many infants). This can appear 3 to 30 days following ingesting spore-containing honey (The AAP Red Book, 2000). Typically, the parents then observe increasing listlessness, decreased appetite, and weakened cry over the next several days. Nursing mothers often report new engorgement. Sometimes this is the full extent of the disease. If the disease progresses, however, the child moves less and less and might begin to drool from the mouth. Gagging and sucking reflexes diminish. Loss of previous head control is also an important sign. Complete respiratory arrest can occur either suddenly or gradually.
If an otherwise healthy baby develops constipation, followed by weakness and difficulty in sucking, crying, or breathing, then infant botulism should be considered the most likely diagnosis until proven otherwise.
The single most effective way to prevent infant botulism is for infants to avoid honey. Breast feeding also appears to lessen the severity of botulism cases.
Therefore, despite other health benefits, honey is an unsafe food for any infant. HONEY SHOULD NOT BE GIVEN TO CHILDREN YOUNGER THAN 12 MONTHS.