If you're planning to use a new Nexus 6P or Nexus 5X with your existing Project Fi service, but didn't order the phone directly through Project Fi, you'll want to order yourself a new Fi SIM card. For whatever reason (it still isn't completely clear why), Project Fi is rolling out a new kind of SIM, and it's required if you plan to use one of the new Nexuses with the service.
If you have an active Project Fi account and have been using it with a Nexus 6, head over to the SIM order page at fi.google.com/order-sim and order yourself a new one. The SIM is free, as is standard 3-5 day shipping, and when it arrives you'll be able to quickly activate it (replacing your old SIM) in the new Nexus and be up and running in no time.
Likely due to a flood of demand from the new Nexus launch, the new SIMs are rather backed up on the order page. For that reason we recommend ordering one as soon as possible so that it can arrive around the same time your day one Nexus orders do. Project Fi is quoting a delivery range of October 29 to November 2 for standard shipping, and October 26 to 27 for overnight shipping (which is $15.99 plus tax).
If you're just starting out with Project Fi and ordered a new SIM recently in anticipation of your new Nexus arriving, you're likely okay — but it's best to double check with Fi support to make sure the SIM you have is a "new" model. At the same time, if you ordered a Nexus 6P or 5X through Project Fi's own store, you don't have anything to worry about.