Airports will change US dollar, Euro and sterling traveller cheques - El Dorado airport in Bogota offers a reasonably competive exchange rate however most Colombian banks will only change US dollar travellers cheques. Hotels in Cartagena will change travellers cheques and foreign cash however at a less desirable rate. Credit cards are accepted in all major shops , hotels and restaurants in Cartagena and Bogota however overseas visitors will probably be asked to provide some kind of ID (such as a photocopy of their passport) to allow a credit card transaction to go through.
Most banks make a charge for your overseas ATM withdrawals, so it makes sense to withdraw the maximum amount out of an ATM. ATMs seem to vary in the maximum single withdrawal from 300,000 = $USD 140 (most common), to 400,000 (Bancolombia) and one ATM in Cartagena which provides 500.000 (~230 USD) which is on the "Plaza de la Aduana" in front of the mayor house in the Bank DAVIVIENDA. Locals will tell you that they can make a maximum of 3 withdrawals of 400 000 pesos per day, but this seems to be in accordance with their own banks' regulations. UK banks seem to permit a maximum of only 2 of these withdrawals (eg 800 000 max per day).
Warning to UK travellers: GB Pounds are not widely accepted for exchange in banks - Money exchange offices which do accept UK sterling commonly offer very very poor exchange rates for cash (eg 3400-3500 pesos/£) rather than 4200 which your ATM will offer (effective March 2007), leaving you 20% worse off if you insist on changing cash. That said, the money exchange office in Bogota airport was offering 3990 pesos/£ as of March 2007.
A rough exchange rate is COP4200 = GBP1 or COP2200 = USD1 (March 2007)