Having contracted on Verizon network programming projects, I can say that they take great pride in being up the traditional 99.999%. If something did go wrong once (and after all, who could predict the popularity the first time the iPhone went on sale), then Verizon would've never let it happen twice.
That said, I believe the activation holdup the first time in 2007 was because of a third party hired to provide the servers for it. Since Apple and ATT hired them, they held joint blame for any goofups.
But since 2008, I'm pretty sure the holdups have been in Apple's iTunes servers, not ATT.
In any case, it doesn't exactly give warm fuzzies to the idea of Apple handling mass notifications.