Yeah, I misspoke. It's a card not a fob. Like others said, it isn't a valet/guest card. It is actually the main "key" to your car in addition to your phone.

Oh, geez. Did they at least make it so that you can have more than one phone associated without sharing the same password?

Yes, it supports three according to the manual.

Wow, Tesla used a whole two bits to track the number of associations!

Does this work when your phone runs out of charge, or do you need to find a good samaritan that will charge it for you?

The key card is your backup for when the phone isn't available for whatever reason. It's not a valet key, it's the main car key. (Valet mode is activated from the touchscreen, or your phone.)

Personally, I'm looking forward to not carrying a bulky fob with me everywhere. The key card will go in my wallet with my other cards, and my pockets will be lighter.