With the Curve, you can use wifi if you don't have a data plan. Does this work the same way for the iPhone? I've heard people say that AT&T forces you to get a data plan in order to use a browser.

You can't do much of anything without an AT&T data plan on an AT&T iPhone 3G. At this point you can't unlock it either.

So even though the iPhone has wifi, it's pretty much useless with out the data plan?

I just took the SIM card out totally. WiFi works. In other words, it is a iPod Touch (which also has WiFi).