Wind roams on other Canadian providers when you step off their footprint, but they are not an MVNO. They are a real carrier using the AWS1700mhz band. You pay roaming when you are not on Wind.

Two American roaming SIMs have pretty good rates in Canada. Take a look at Pirahna Mobile and Telna. Pirahna's rates are .22 cents a minute outboud to the US; .13 a minute inbound on a US number, 16 cents an SMS (outbound), free inbound SMS, .40 a meg data.