I just posted on the other T1 thread about going landside. It will really be a problem coming off an International flight, or from a connecting Canadian flight, going to the US and you have checked bags, since you are channeled into a separate US Pre-clearance area where your checked bags will come down. You would have to convince the US agents to let you into this area when you enter through the channel that Toronto-originating passengers do. Given their general temperment, you may not want to risk this...

As for arriving from the US and going onto an International flight, even with checked bags there'd be no real problem since the bags do not clear Canadian customs and go right onto the International flight (as is the current case). Again, its a matter of getting an Immigration officer who doesn't raise any concerns with a "2-hr visit" to our country.

As for International to International...good question. It wasn't among the scenarios I saw played out, so I can only speculate, and I'd rather not. You should check the GTAA website as I am sure they will provide these transfer scenarios. Also, given AC is trying to promote itself as a major transit carrier for carrying people between continents without the hassle of US visas, etc. I would expect the procedure is much like US to International -- i.e. checked bags go right onto your next flight with no need of Canadian customs clearance. Coming landside would be an option as you'd just clear Immigration and then re-enter through security and follow the course set out for Toronto-originating passengers. However, I would expect there would be a cursory documents inspection upon arrival, and you would be channeled through the 2nd floor corridor, likely with a "satellite" Canadian immigration station screening, after which you'd cross over a "bridge" and head back into the International area in the hammerhead, or to the MLL.