I believe the main issue with Canada up to this point is that the combined residencies are not technically ACGME accredited (the seperate programs are but the combine program itself isn't accredited).
For Med-Peds, the Medicine was accredited, the Peds was accredited, but Med-Peds itself was not accredited - it just had the blessing of ABIM and ABP. Starting 2007, Med-Peds must be ACGME accredited. The ACGME is starting with med-peds since it is the largest of the combined residency, but eventually it will turn to other combined fields.
How will this affect Canada, I'm not sure.
A quick search on the internet have shown that some Med-Peds graduates are able to obtain positions in Canada (ie Adult ID in Winnepeg, hospitalist in Toronto, etc) so it may be possible to with IM/FP combined program. But your best bet is to inquire with the authorities.