I chatted with Verizon support via Twitter, and this is the information I received...

• As already noted, with Verizon's Unlimited Ultimate plan, after exceeding 10 GB/mo of high-speed international data, speeds are reduced to up to 256 Kbps for the rest of your monthly billing cycle.

• Data Boosts are *not* available for international data.

• However, for international data, Verizon has something very similar to Data Boosts called Speed Boosts. If you exceed your monthly limit of high-speed international data, you can purchase Speed Boosts. Each unit is $10 and includes an additional 10 GB of high-speed international data.

• Things work slightly differently for Canada and Mexico. I can't remember the exact details.

• You can monitor your international data usage in the Verizon mobile app.

If this is not consistent with anyone else's experience, let me know! But this is the information that came directly from chatting with Verizon.

Also, just want to note that planes&trains had some good advice above about using Verizon abroad: "In two instances (Japan and India), I was initially not too thrilled with the speeds. I could turn off auto-join and find a provider I knew would have better speeds and could connect and continue without a problem. So, it appears one is not tied to a single provider."