The latest iPad iOS 4.2 Beta 2 released yesterday appears to include new tethering options in the settings (pic via Paulgrav). According to 9to5Mac, the options only appear on European carriers and do not yet appear on AT&T devices.
As best we can tell, these settings might allow you to tether your iPad 3G's internet connection to another device, such as a laptop. This means you will be able to share your iPad 3G connection with your laptop (via a USB cable). You can similarly share your iPhone's 3G connection in the same manner, though AT&T charges an additional fee to enable this service. Unfortunately for iPhone owners, this still doesn't allow you to share your iPhone's 3G with the iPad.