You can drive in the UK for up to 12 months on your US driving licence, however in that time you will have to apply for a provisional licence, complete the driving test and driving theory test. If you don't do this within 12 months you will be regarded as an unqualified driver. There's no mechanism for the automatic transfer of licences from the US.

See http://www.direct.gov.uk/en/Motoring/Dr ... DG_4022562 for info

Just to update (for future reference) direct exchanges of foreign licences for British ones are only available for licences issued in:
Northern Ireland (NI)
European Community/European Economic Area (EC/EEA)
Jersey, Guernsey or the Isle of Man
Gibraltar, Australia, Barbados, British Virgin Islands, Canada, Falkland Islands, Faroe Islands, Hong Kong, Japan, Monaco, New Zealand, Republic of Korea, Singapore, South Africa, Switzerland and Zimbabwe.