I asked Virgin Mobile USA today if, assuming I bought one from the Google Play Store, I could activate the new Nexus 5, and this is the response I received on Thu, Oct 31, 2013 at 9:44 PM:

Thanks for contacting Virgin Mobile Customer Care.

We want to let you know that is not possible to activate a non-Virgin Mobile phone like an unlocked one because our system only recognizes the unique serial numbers that our phones have. You will need to purchase a Virgin Mobile phone.

You can check our great phones and prices by clicking the link below: http://www.virginmobileusa.com/shop/cell-phones/

If you need additional assistance, feel free to let us know how we can assist further or contact us at 1-888-322-1122 (or *VM from your handset). You can reach us from Monday to Friday 6am -10pm and Weekends 6am -9pm. As a reminder, please include your Virgin Mobile phone number and PIN on all replies.

This is interesting, because as far as I know, Virgin Mobile actually uses the Sprint network, and Sprint is one of the carriers that will support the Nexus 5.

That’s right, the Nexus 5 will be sold through Sprint and T-Mobile, with only Verizon Wireless opting out of carrying the device. Updated at 3:26 p.m. PT: Google came back and confirmed that AT&T won’t be selling the Nexus 5, but it will be able to run on its network.
http://news.cnet.com/8301-1035_3-57610241-94/with-nexus-5-google-finally-gets-flagship-phone-treatment/

So, it looks like I’ll be jumping ship to T-Mobile after nearly two-and-a-half years as a satisfied customer of Virgin Mobile.