Many thanks to readers Anne Marie and Jodi J. for the tips about this:
Per the NYX website, the five NYX stores (all in California) will be in Walnut Creek (Holiday 2015), Santa Clara (Holiday 2015), Arcadia (Oct. 2), Rancho Cucamonga (Nov. 6) and Torrance (Oct. 9). I've seen references to there only being five stores, so I don't think more are on the way in the immediate future (but let's hope there are more eventually!).
If you live near any of these cities, and you visit a store when it opens, you KNOW I would love to hear from you about your experience. The closest store to me here in San Diego is the one in Rancho Cucamonga (Google Maps tells me it's a little less driving time than the one in Torrance) but it may be a while until I can make it to a store myself, since it's about a two hour drive, one-way.
Thanks again, Anne Marie and Jodi J., for the great tips!