Sometimes its worth buying a mobile with a different operator and paying to have it unlocked. The difference in price they charge for the same phone can be loads.
Google blackra1n and blacksn0w, it's easy to jailbreak and unlock a 3GS. Jailbreaking allows you to run non Apple authorised applications on the phone which in turn lets you install software that will unlock it to any carrier.
Check your serial number though because its not been perfected yet for certain models and on those models you cant turn your phone or ipod touch off or you will have to do it again.
all i know is its the 3.1.2 version im getting it tommorow its the 16gb 3gs in white wot shud i be paying plz for a model like this its 1mth old in a1 condition .Thx for all your help ...