You'll probably have to pay around $10 to add cellular for the watch, similar to how it works for the iPad plan.

Also curious if it's available to people on obscure prepaid plans. I'm on T-mobile's $30 unlimited data+texting / 100 voice minutes.

Not looking at a new smartwatch until my pebble dies, and not that excited about $70 for built-in LTE (especially if there are ongoing monthly charges), but I'm curious.

That's going to be up to your carrier. In the US AT&T and T-Mobile seem to be charging $10 for watches, and Verizon charges $5 if it shares a phone number or $20 if it has it's own.

Piggy-back. You do not get a separate phone number for your watch. They will charge you $5 or $10 a month for the 'wearable device'.

Define 'new line'. It'll piggyback on your phone's phone number at an additional charge (up to the carrier)