Here's the BAA leaflet with the general restrictions (for all destinations):
http://www.heathrowairport.com/assets/B2CPorta l/Static%20Files/hand_baggage_21Aug.pdf
However, the BAA website adds this, about flights to the USA:
"Extra restrictions are in place. Passengers will be subjected to secondary security searches at the gate. No toiletries or cosmetics purchased in the departure lounge will be allowed into the aircraft cabin and any drinks or liquid items must be consumed before boarding. Food, however, is allowed. Passengers boarding flights to the USA and items they are carrying, including those acquired after the central screening point, will be subjected to secondary search at the gate and any liquids discovered will be removed.
If you are in any doubt about what you can and cannot take on board please ask a member of staff."
Chris