That's a different question.
Not too different of an answer though. You still need a load balancing driver to make it possible for the system to see AND use 2 internet connections at the same time. This could theoretically be done on the PC, as you can write drivers for the PC.
But the upside of doing that would hardly be worth it (3G is incredibly slow compared to your Wifi/broadband).
If you're just talking about tethering, so you have internet access when you're away from your wifi, then that's a different story. I think most iPhone providers either block tethering completely or require you to buy a tethering package. There are jailbreaks that bypass this. (I've used Android for the last 2yrs, and lost touch as to the latest for iPhones + tethering). But I've got a feeling this isn't what you're referring to...