According to GSMworld.com, Sasktel will be starting to switch its network to GSM, the same technology Rogers uses. MTS doesn't appear to be going GSM anytime soon. Which means that MTS is the only "major" CDMA carrier in all of Canada that has no plans in the near future to adopt GSM. I'm not sure how well Sasktel will do though since they will be using the 1900mhz band, which will not do much use for rural coverage (850mhz will be needed for larger areas to cover).
http://gsmworld.com/roaming/gsminfo/net_casa.shtml
I hope GSM coverage comes sooner rather than later with Bell, Telus and Sasktel (as well as MTS). I would love to be able to use my Rogers phone in the boonies