I was reading in an article recently about the precautions people need to have in using these old wooden pallets. It stated that "recent regulations designed to prevent invasive pests from hitching a ride in the wood means some pallets really shouldn't be used in the garden or in the home or even burned for fuel either.
Pallet manufacturers in Canada and US must now either heat-treat or fumigate pallets as a precaution against such pests as the emerald ash borer. Treated pallets are marked with the International Plant Protection Convention logo and letters indicating what type of treatment was used. Pallets with "MB" in the logo have been fumigated with methyl bromide, a poisonous, ozone-depleting gas that has been phased out for most agricultural uses. Pallets marked "MB" should never be used for gardening or fuel, while those with the letters "HT" were heat treated or kiln dried and are safe to reuse.
Even so, it’s wise to approach pallet reuse with a degree of caution. Wood is porous and may have been exposed to chemicals, bacteria, or unsanitary conditions during shipment. For garden use, seek pallets that have not been used to ship toxic products or food. Wash any pallet with soapy water or bleach before using." (excerpted from June/July Organic Gardening Magazine).