http://www.a1gp.com/News/NewsArticle.aspx?newsId=42050 There we go - now they mention Canada and clarify the rule - you can only RACE for 1 nation, even if you test or practice for others.
(This isn't technically true, of course, as Italy's Enrico Toccacelo drove for Pakistan in season 1 at Durban as an emergency replacement for Adam Khan - however it was agreed that Enrico couldn't score any points
)