Originally Posted by
Prospero
As you'll need to fill out another darned I-94W visa waiver on arrival in MIA the next day, I'd be inclined to enter "in transit to Cancun" when you first arrive into SFO and the address of your hotel in LAS for immigration in MIA.
You don't actually need to do another I-94W if you're doing a side trip to Mexico - you just retain the departure card. Saying that, I did fill another one out as I couldn't be bothered trying to explain why I already had a departure record in my passport.
Cheers,
Rick