Transferring an old iPhone after I upgrade?

I'm about to upgrade from the iPhone 3G to the iPhone 4, and I'm not sure what will happen to my old device once my phone number is transferred.

Specifically, I want to give the old device to my girlfriend, who is on a AT&T non-smartphone. What will I have to do to move her to the iPhone 3G? Will she need to sign up for an iPhone specific data plan if I put the sim card in there? WIll it work like any old phone if I just put the simcard in there?

In short, I'm not sure what will happen, and what I need to do.

I just did this myself - I upgraded to an iPhone 4 and gave my sister my iPhone 3G. I put her AT&T SIM inside and she was instantly able to send & receive calls and texts. Since she doesn't want to pay for data (yet), she used the new iOS 4 network setting to turn off cellular data. I have heard that AT&T scans their network and automatically adds data plans to 'smartphones', but so far this hasn't been the case.

If she does want to use data, have her call up AT&T and let them know about the phone switch on the plan and they can sign her up for the iPhone data plan - it's $15/month for 200MB and $25/month for 2GB. You may be able to do this yourself online - I haven't tried - but contacting AT&T is quick and relatively painless and is guaranteed to work.

One other thing - if her SIM is ancient (pre-3G), then you may need to get a new one that supports 3G from an AT&T retailer or call AT&T and they can ship you a new one. They may charge a nominal fee (~$10) for this service.