Turns out one of my nephews has been interested in this question and has researched it some though I gather not definitively.
My father, his grandfather, was naturalized before he married my mother so whatever his citizenship status was wouldn't automatically come to his children, but according to my nephew it should nevertheless be a lot easier for us to get dual citizenship if we want it.
As others here are saying, you do have to renounce your Canadian citizenship on becoming a US citizen, but Canada doesn't recognize that renunciation.
According to my nephew, his generation doesn't get any special benefit in seeking dual citizenship through their grandfather, but if my brother decided to do whatever it takes and get dual citizenship, then it would consequently be easier for his children to get it too. My brother goes to Canada whenever he gets the chance because he's a Canadian in spirit and loves all our multitudinous Canadian relatives, so I could see him doing it.