Using AT&T GO (prepaid) SIM Cards in iPhone

[The-Brit]

Firstly, apologies if this topic has already been discussed - I didn't see it in any searches.

My wife currently has an (oldish) Nokia AT&T 'Go Phone', which she rarely uses. Having recently bought her an iPod Touch, she is now 'hooked' on the interface and was asking whether she could upgrade her current GO Phone to an iPhone.

Technically, assuming I can acquire an 3G iPhone without paying for the service, could I just drop the SIM from her GO phone into the iPhone and she could continue to use it? (I suspect the answer is no, but I'm curious to know if there is a work around?)

Thanks in advance.