I’m on the train in to Philadelphia and writing this post on a wifi-only iPad via wifi tethering. The 3G iPads come out later this month, but why would you want an additional $30 per month plan when most mobile phones have decent bluetooth or wifi tethering options?
I set up my Google nexus one (T-Mobile) a couple weeks ago for wifi tethering. One tap of a button and it shares its 3G. The iPad connects through it seamlessly.
If you have a tethering ability with your phone, try using it before opting for the 3G iPad. You can save some skrilla and have a little longer battery life (on the iPad – mileage on your phone may vary).