Tethering is not supported on the iPad without jailbreaking as jhyiesia mentioned. The iPad does support VPN via 3G although it has only been tested with the OSX Server's VPN. It may be worth a try.
You're best bet seem to be to use one of the newer Verizon smartphones that support mobile hotspots via an app such as the Palm Pre Plus which can act as a mobile WiFi hotspot for up to 5 different devices.
If Blackberry is your only option, they still don't have a WiFi app like the one on Palm's Pre but that may change.
Good luck.