Right now this is the rule for travel from the UK.

New Carry-On Baggage Restrictions FROM The United Kingdom
Effective immediately all carry-on baggage must be processed as checked baggage for all flights departing from airports in the United Kingdom (U.K.). Due to these changes, American Airlines will waive excess baggage charges through August 17, 2006, for those passengers who need to check their carry-on bags due to these new restrictions, up to a maximum of two excess pieces.

Nothing may be carried in passengers' pockets. Passengers may only take the following items through the airport security checkpoint, in a single transparent plastic bag:
pocket size wallets and pocket size purses plus contents (money, credit cards, identity cards, etc. but not handbags or purses)
travel documents essential for the journey (passports and travel tickets)
prescription medicines and medical items sufficient and essential for the flight (a diabetic kit for example), except in liquid form unless verified as authentic.
eye glasses and sunglasses, without cases
contact lens cases, without bottles of solution
for those travelling with an infant: baby food, milk (the contents of each bottle must be tasted by the accompanying passenger) and sanitary items sufficient and essential for the flight (wipes, creams, disposal bags)
female sanitary items sufficient and essential for the flight, if unboxed
tissues (unboxed) and/or handkerchiefs
keys (but no electrical key fobs)
If an item does not appear in the list above it is NOT allowed. Additionally, no liquids, gels or other items of similar consistency are permitted to enter the sterile areas except for a) baby formula, breast milk, or juice if a baby or small child is traveling; b) prescription medicine with a name that matches the passenger's travel documents; c) insulin and essential other non-prescription medicines. This also includes liquids and gel products purchased as duty free.

All U.K. transiting passengers will be required to check any carry-ons they bring to the airport regardless of origination. Passengers who have already departed the US and will be connecting in the UK will not be allowed to use the transit area in the UK airports and will be routed back to the ticket counters to check their carry-on luggage.

This comes from the American Airline website.