Update, because I know people hate it when posters ask questions and never come back.
I bought a $13 Hall sensor from Amazon, hoping it was that, only to take it apart tonight and find out that the plastic teeth on the rotor were all worn off off. It looks like the bolt holding it on had loosened a bit (came off really easily)...
Anyway, I Googled a bit and tried to find a replacement hub but it seems like the thing is just sold as a $50+ unit. Is there a source for new hubs? Or is replacing the whole shebang the only way to go?
I was expecting this thing to be super-complicated inside, but after watching a few videos it seems pretty simple aside from the electronics.