It depends on the store, here in Seattle, there are 2 stores where silicone cases are available. Update: Just ordered a green silicone case at Southcenter to pick up this evening. It would take a few weeks to get one through the mail.
Out in the Bay Area, I’m starting to see “available in store” updates via their apps. I had placed orders for the MagSafe, but it wasn’t going ship until November, but it shows availability at the store for pick up. So I believe they’re getting their shipments for the preparation of the release on Friday.