Verizon/Sprint Mi-Fi's, using MyWi on an iPhone, and WalkingHotSpot on a WinMo are all great, but at the source, they all require a contract tied to a device. The iPad 3G has a stop-and-go 3G data plan.
Now, if you could activate the iPad 3G's MicroSIM, then take it out and put it in a Micro-SIM to SIM adapter, then stick it in a UMTS Mi-Fi device such as the NovaTel MiFi 2372, now you're talking the best of both worlds...
Let's just hope AT&T doesn't kill the MicroSIM for breaching their ToS.
If you could do that, then I would have to consider it...