Would there be a problem with using a US-based credit card in getting airline tickets to hvn from canada or mexico?

There won't be a problem if you use a Canadian travel agent.

Thank you for the help guys ^ ^

Looks like it'll be a while before I visit Cuba, if at all.

My friends from Virginia..flew through YYZ...
 

...however one issue to consider is when you are returning home from your winter trip to "Canada" how do you explain that amazing tan? They got a way with saying they were skiing all week in the sun...but be prepared to get the question. The real value of Cuba is to go to an all inclusive which are very cheap from Toronto. My friends just used their points IAD-YYZ and then booked an all inclusive using a Toronto travel agency. I have to say its one of the few things thats cheaper in Canada than in the US is all inclusive vacations, simply because we have charter airlines that service this business exclusively. Also you do not get a stamp in your passport, the only penalty is the 10% surcharge when changing $US. Have fun...Cuba is a nice place filled with very friendly people.

I've heard rumours of U.S. immigration/customs agents at YYZ somehow observing pax disembarking from Cuba flights (with the unofficial assistance of some Canadian customs/immigration people?) and then watching out for these pax who then subsequently preclear.

What I'd do, if I was an U.S. citizen, would be to book the cheapest all-inclusive in Mexico that I could find, just toss that whole stay, and fly to Cuba from Mexico (paying cash for the Mexico-Cuba tickets).

That might be a bit dangerous to do. It depends on how the flights are ticketed. You could be in trouble if say, you book on Cubana and the tickets are made as a charge from Cubana rather than the TA. Similarly, the ticket number could end up on your CC and could be traced if the government has it in for you.

How about canadian immigration people - what do they say when an American arrives from cuba?

Officially, it's none of their business. They should screen as they would any non-resident/citizen coming from anywhere else. However, there may be some officials who may agree with the U.S. embargo on Cuba and as a result report U.S. travellers to U.S. agents. Again, this hypothesis is based on rumours and other possible scenarios.