The short answer is yes

If the phone is unlocked you will have no problem. All iPhones sold in NZ are factory unlocked (not locked to a provider). The fact you are using your iPhone on 2degrees means that it is unlocked.

An unlocked iPhone can be used anywhere in the world where there is a compatable cellphone network.

The iPhone 3GS uses the following GSM bands

UMTS/HSDPA 850, 1900, 2100 MHz (good for phone calls and high speed data)

GSM/EDGE 850, 900, 1800, 1900 MHz (good for phone calls and low speed data)

AT&T will definitely work as a carrier, the iPhone was designed specifically with them in mind. AT&T uses 850, 1900 MHz bands.

T-Mobile is the only other major GSM carrier in the US (to my knowledge). Unfortunately it appears that they use 1700MHz for their 3G data, which means you're stuck with low speed GPRS/EDGE data.

IMPORTANT:

As you will be there for so long, there is a good chance you will want to upgrade your phone. If you buy an iPhone from the US and intend to use it here, make sure it is factory unlocked. If you buy a locked iPhone, it may be no more than an  iPod outside the US.

At this point in time, all the cheap deals on iPhones are for phones locked to AT&T. Unlocked phones are available, but they're not as cheap.

There are ways to unlock an iPhone, but they are far from fool proof. In my opinion, it's not worth the hassle.

Have a great time.