Although my landing paper had already been stamped, I had a 10-year B1/B2 visa to US on my chinese passport, the visa expires in 2005. So, I went into China with my chinese passport, and when I left, I told the boder guys that I would go to US, just stopping by Canada, and they let me go without any problem. For later trips, I used canadian passport, along with the visa I got from chinese embassy(which cost company 76 bucks :)). I think it's safer to use your canadian passport with a valid L
visa.