Actually, this question only came up quite recently, and we hadn't considered it before. As a result of subsequent discussions, we've learned that the U.S. hadn't considered it before either. But having received the question, it was good to try to sort through the answer.
The second thing I will say is that, yes, U.S. citizens are subject to U.S. tax, which means that if you are a U.S. citizen resident in Canada, you're essentially exposed to both the Canadian and U.S. tax systems.
To get to your point about RESPs and RDSPs, we do not tax the government grants when they go into the plan. That's kind of antithetical to the point. But when those grants or other income generated by the plan do come out of the plan in the beneficiary's hands, they are subject to Canadian tax.
Ideally, the U.S., if they chose to tax it at all, would try to match that taxation to tax it at that time. So we would tax. If there is citizenship tax on top of that, there rarely would be, but if there were, then the timing would be right.
We had the opportunity to discuss this with the U.S. since the question has been asked of us. As I said, they hadn't considered the question before, but based on the description of the plans we provided them, they told us that indeed they consider there to be no taxation at the time of the grant or other contribution to either of these types of plans, but when the amount comes out and represents income to us, they would consider that it would represent income for U.S. purposes, too. So there'd be taxation at the same time.