I got a call and an e-mail today. The e-mail notified me that I'm on the reservation list. In the call, their representative said that the Powerwalls (I reserved 2) should be delivered in the next 1-2 months, and that I'll be contacted about working with a Tesla licensed installer. When the rep saw that I was in California, he revised his estimate to "about a month".
I have two separate SolarCity installations (done about 2 years apart, each leased for 20 years). I'll likely get one wall installed on one PV system, and one on the other. Neither inverter that is installed would be able to handle a full 10 kW from the combined output of the two Powerwalls, so I suspect they'll be split this way. I'm not sure what kind of load I might have that would require a constant 10 kW (charging multiple vehicles + running A/C?), but redundancy seems like a good thing to me.