Just went through the YYZ transborder connections. They have a 3 Step process:

Step 1 - Self serve - Scan in your boarding pass, indicate whether or not you have a bag.
Step 2 - Skipped this, I had carry-on.
Step 3 - Present documentation to YYZ agent before entering U.S. customs hall.

It looks like they are finally checking bags through to your U.S. destination without you having to collect and re-check in.

I talked to the agent at Step 3 and she said it's been up for a few weeks and that the baggage carousel will eventually be removed.

I like the self-serve boarding pass scanning. This may mean that Nexus cardholders may be able to continue without taking to an agent. This used to be the case with YVR until they did check-through baggage and then you need to talk to a customs agent so he can release your bag.