Welp, nameservers are down at my old company.
My initial theory was that my old boss accidentally allowed the primary domain to expire, because whois says they're set to expire on 02-23-2014 -- suggesting maybe they expired on 02-23-2013 and he just renewed them and we're waiting for propagation. But then I noticed it also says the record was last updated on 04-08-2009, which I think means it was renewed all the way through 2014 back then, meaning the date is just a coincidence. (Also, the whois gives IP's for the nameservers and they're not responding to ping, suggesting it's not just a DNS resolution issue.)
On the one hand, I am so, so glad that I don't work there anymore.
On the other, I like to think this is not a mistake I ever would have made.
On the third hand, I did in fact send him an E-Mail (through the secondary domain, which is still functioning) and tell him I'm available if he needs an extra set of hands.
Anyhow, my domains are down for the moment. And so are all the other customers'. Again -- glad I'm not the guy having to deal with end users. (Aside from my Web development clients, who let me know there was a problem in the first place.)