[GadgetKen]

Think multiple ID sim is like the ekit simple calling sim card that has a UK and a US number assigned to it(Andy correct me if I'm wrong). Don't think would work well for an iphone though because while technically feasible, you wouldn't want to pay by the megabyte for data usage.
The iphone 3gs is a nice phone but is a bit of a data hog. It's possible to do AT&T prepaid with 100 mb data packs at $20/each...originally it was unlimited but some people got a bit overenthusiastic with data usage. Used to run an AT&T Tilt this way but I can't guarantee an iphone would work in the same fashion.
Some thoughts:
Get an ipad (I think they have moderately priced prepaid service packages)
Use a USB key on a MacBook. You can get month to month (I've got Millenicom/Verizon but they also represent Sprint and T-Mobile) or prepaid as you go...Sprint/Virgin Mobile is a nice value (varying packages up to $60/5 gb) and can stay dormant up to 1 year after last activation. AT&T and Verizon also have prepaid service now but it's a bit pricey compared to offers through resellers.
Unlock the iphone and use it on T-Mo with one of their no-contract options.