Hey Jessy,
welcome to the forum.
i'm pretty sure your original iPhone SIM will work in the 3GS. When I got my 3G i just put my original SIM in and it was fine. Given that o2 say you can do this with the 3GS aswell there should be no need to get a new contract SIM sent out to you.
i'm thinking of getting the 3GS and putting in my contract SIM and continuing on a lower tariff til my contract expires in Jan. then i'll have a good look at what tariffs are on offer from o2 at that time, free from considerations of subsidies. also by then we'll know exactly how/if the 3GS functions on simplicity etc (ie no visual voicemail, tethering...)
Although i can get over £300 from envirofone for my 3G together with an old Nokia n95, i'm still gonna wait for the product tear down/reviews/un-boxing to make sure the camera is up to scratch before taking the plunge. so i wont be getting up early on Friday

(unless the reviews are breathtakingly good)
@April Dancer, Jessy is right you can leave the PAYG SIM in the envelope there's no obligation to use it. having said that, it would be a waste of the 12 months free wi fi access if you had another handset which could benefit from it.