Not getting the iPad in either version yet. Gotta hold it and use it before I decide. That said it took me 30 seconds to decide I wanted an iPhone when they first came out. However, as far as WiFi only vs. 3G, I thought this really was a tough one, but then I realized, 3G everything on AT&T is actually painful, better than EDGE, but still painful. Not worth my extra $130, or the $15-30 monthly cost. I would have said get the 3G and only sign up on months when you absolutely need it, but nah, get the WiFi, and if you need web access when you don't have it on the WiFi, use your iPhone. If people can JB their phones and tether the iPad successfully without any problems, expect JBing on iPhone to rise.
OR
If it really is that simple to cut down your 3G Sim to a mini, put it in an adapter tray when it is in your phone, but then power down and slap it in your iPad for a little while, as long as AT&T doesn't know/notice, then you will see a lot of that and that might be the reason to get the 3G version. People aren't paying $45-60 a month for data.