The same thing that occurred to customers in the UK has now occurred to our friends over the water. Full details are on the Twitter Blog, however I suspect this is due to the costs associated with maintaining the sms service as Canda, like Europe does not charge end users to recieve SMS.
However there may be some good news in the future on the twitter blog;
There is a realistic, scalable SMS solution for Canada (and the rest of the world.) We’re working on that and will post more details on the Twitter blog as we make progress.
UPDATE- Krystal (our resident Canadian) has informed me that you do pay to receive SMS in Canada