Android :: Take Micro Sim Card From AT&T?

I have accepted that I just can't get a smart phone. I'm on VZW and have a enV touch, its driving me up a wall. (I also made it go swimming with me so that's problem why). So to cut things short, is it possible to take a Micro Sim card from AT&T put it in a adapter and put it in say, a nexus one or captivate? What id do is take the no-contract pre-paid data plan of the iPad 3G, most likely the 200MB/month for $15 and use Google Voice and other various VoIP clients to use as phone and GV for texts. I pretty much have WiFi where ever I go and pretty decent AT&T 3G in my area. Yes I know, it's just not that simple. Id have to change the APN on the device to att's broadband. Tweak a heck out the phone. 1.) Is it possible? Ive seen it done for a data only iPhone 2.) It is legal right?