To James: SIN does expire for non-Canadians, but even if they are expired and you keep working wether for the same employer or not, legally or illegally, government will still deduct your taxes using your SIN number. I hold a restricted work permit (I'm only allowed to work for a certain employer at a certin location), we submitted our app in March together with my OWP app, that puts me on an implied status. I can no longer work when my work permit expires UNLESS, my employer will renew my contract and start with LMO process all over again. IEC must be different from what I have right now, but like what it says on the letter you posted, you can continue working based from your work permit restrictions.
And update for the drivers licence: Here in Alberta, wether you have an international drivers licence or a Canadian licence, they will still ask for immigration papers if you don't provide the registry with a Canadian passport. I was there yesterday, I have everything.. NS drivers licence, a piece of mail and my non Canadian passport... The lady asked me for my PR card or citizenship card. When I told her my case, she said no I can't hold an Alberta drivers licence until I get my OWP or PR card.
... 
There's always next week though.
