As much as I hate the outrageous high roaming charges, I'm not sure regulating them completely away is the best idea. Dictating companies how much and for what they can charge is one of the strongest possible market intervention, and may very well lead to less competition and therefore, higher prices and (eventually) less innovation.
Personally I think the same effect could be achieved by a much smaller intervention, namely decoupling billing from physical SIM cards. If it was possible to get the local (prepaid) rate without having to get a local SIM card (which, even in 2013, is often a somewhat difficult procedure, especially if you don't speak the local language), there would not only be no reason for roaming charges any more (just get a cheap prepaid rate and you are set), but there would also be more (not less) competition, because one could switch providers any time, even in his own country. The phone number, of course, should remain bound to the SIM card, just the billing should be "virtualized".
Personally I think the same effect could be achieved by a much smaller intervention, namely decoupling billing from physical SIM cards. If it was possible to get the local (prepaid) rate without having to get a local SIM card (which, even in 2013, is often a somewhat difficult procedure, especially if you don't speak the local language), there would not only be no reason for roaming charges any more (just get a cheap prepaid rate and you are set), but there would also be more (not less) competition, because one could switch providers any time, even in his own country. The phone number, of course, should remain bound to the SIM card, just the billing should be "virtualized".