Originally Posted by David-A

N.b. some payment cards issued outside the UK, espcially in the USA, may not work for contactless travel (i.e. as an oyster substitute) - even if the do work for contactless payment when purchasing tickets or topping up oystercards at machines.

http://tfl.gov.uk/fares-and-payments...on-this-page-1

Also, people should be wear of overseas transactions fees. Check what their card charges.