I'm back from Canada! To my knowledge, there were no disasters of unwanted data or call charges, though I'm not going to be completely happy till I've received my bill for this month.
I'm using an EVO 3D, and it had good protections from unwanted international or phone call data use. When I first arrived in Canada and turned the phone on, I thought something was wrong because I got no phone signal whatsoever, even in areas where my host told me other US friends had used Sprint successfully. I checked through the menus carefully to ensure there wasn't something unchecked, and found nothing. The next day, I took the phone into central Montreal where I was sure there would be a signal if this was going to work anywhere, and still had nothing. I checked the menus again carefully, and now I found checkboxes on the menus (I believe they were in the wireless and networks menu, but have forgotten) that I am positive weren't there before! There was one for voice calling, and one for data. I clicked the one for voice, left data unchecked, and voila, I had a signal! I made a couple of short calls, and they worked well. Before each call, I received a prompt warning that I was making an international call and there could be extra charges. I had to select to make the call before it would go through. I'm not sure if these things are in all Android phones, or something in Sense, so I can't guarantee that non-HTC phones do this.
My _BELIEF_ (not positive) is that these checkboxes to turn on international calling don't turn on till something happens, it may be starting up the phone in non-US network coverage, or it may be passage of a certain amount of time.
When I tried to text message, I received a similar warning about international rates, with an option to send the text now, or wait till I was in domestic coverage. It ended up that there were some problems at my work (in the USA) and I used the calling plan more than I expected, though still for probably less than an hour total calling time.
As for the special "Canada calling plan" I mentioned in my original text, I called Sprint CS and set it up the day before my trip, they told me they would have it automatically end the day my trip ended (today). I'm still going to call tomorrow and verify that it is ended.
I left data unchecked, and though I'd done a bunch of things pre-trip to minimize data use, such as turning off syncing on various accounts in the menu, and going so far as to uninstall some apps I thought might grab data, such as WeatherBug Elite, and Taptu (news aggregator), but I think that was probably going overboard.
I used wifi several times successfully during the trip, with all features working as expected. Unless I get a nasty surprise on my bill, I am very pleased with how this worked out.