Tethering is great. So far my carrier has looked the other way so I'm not paying a fee to share data between devices.
AA
Carriers differ and there are big differences in the policies of carriers in the US versus Europe. But having said that, tethering without paying an additional fee may require one to "root" an Android phone or jailbreak an iPhone. (Not always true.) Once that is done, if required, it's likely (at least in the US) that carriers will "look the other way" if you use approximately the same amount of download per month with your phone + iPad as you used with your phone alone.
Think about it this way. The main concern of cell carriers (apart from extracting as much as they can from your bank account every month) is the demand on their networks. Since the iPad (or a laptop) is a much friendlier content consumption device than a phone, there is a strong likelihood that users will consume much more data if they tether those devices to a phone. (Obviously, though, one is not likely to surf the web on both a phone and a tablet simultaneously.) With that in mind, the carriers (at least in the US) tend to look for "spikes" in usage from month to month. As long as you don't exceed your monthly data download ceiling (or show significant increase in data consumption on an unlimited plan) a carrier is likely to be unaware that you are tethering another device to your phone.
It may be possible (opinions differ) for a carrier to determine whether your phone is being used as a tethering device. But even if that's possible, it's VERY unlikely that a carrier will want to bear the PR problems associated with examining whether another device is consuming the data you've paid for. Thus, they concentrate on the volume of data you're downloading, not the device that is displaying the data.
By the way, this same factor is at work in carriers' charges for tethering. For example, Verizon typically charges $20 per month for tethering. But they provide an additional 2 Gigs of downloads in return for this feature. This is the result of complaints to the FCC in the US that VZW was charging an additional fee to use data consumers had already paid for. Of course, if you have an unlimited data plan, adding 2 gigs to your ceiling is meaningless. But by providing the additional download ceiling, Verizon managed to avoid potential penalties from the FCC.