The latest iOS beta firmware has an option under the “Cellular Data” settings for internet tethering, which would allow users to share the iPad’s 3g connection to another device via USB. If it works like the iPhone, then users will have to pay a monthly charge for this feature, depending on the network. (Or, you could just use MyWi to create a free WiFi hot spot.)
Sadly, this option is only available through select carriers – sorry AT&T subscribers. Though, AT&T has only just recently enabled tethering on the iPhone, so it is entirely possible that they activate it on the iPad in the near future. I wouldn’t hold my breath, considering applications like MyWi can do a better job for the fraction of the price.