Recently on CDT/Telecom Digest it was written:
Well, if she's going to be in Canada she really needs to get service in Canada with a Canadian mobile provider. I say this rather than providing a "Canadian" addition to a Verizon plan for at least a couple of reasons. She will likely have people in Canada wanting to call her. Even though it's possible to get good US-Canadian rates it will be much better for local callers to be able to call her without paying either international long distance (which _can_ be but not always is rather inexpensive). You didn't say where in Canada she's going to be but it's likely if she's in a major urban area of Canada she'll have access to Rogers, Fido (part of Rogers now), Bell Canada Mobility, Telus Mobility or Wind (3G only.) Also, prepaid services now offer smartphones with monthly data plans if that's something that she needs.
She can go into one of these providers' retail presence and get service from them or she can also likely go into a major mobile provider store such as London Drug and go to their wireless department and get service. If she's a US national she'll likely be relegated to getting prepaid service unless she has a credit file in Canada.
If she must have a smartphone she could sell it through eBay or some other avenue at the completion of her time in Canada.
Unfortunately, most CDMA providers of which Verizon and Sprint are in the US and Telus and Bell in Canada, do not allow phones that they did not or do not sell and have the phone within their database of MEID (serial number) range. If she presently has US mobile service of course she can use that when she's in the states. If she doesn't want to leave her US service "fallow" while she's in Canada she could also convert her number to prepaid and just make sure that she's got enough paid into the service to keep it alive.
Hope that helps.