*Edit* looks like it's only for the LTE GS3. I just checked on my Wind GS3 (I have both versions) and no update yet.
Looks like the juicy Android 4.1 Jellybean update for Canadian Galaxy S III users is available a few days early. Users are reporting they're getting the 320ish MB update. Just go check manually:
settings -> about device -> software update -> update
I just checked and my GS3 is being updated as I type this.
Go Go Go!
Thanks Treatz, AllanVS and can3gsw!