I think for me it was up updating via OTA. Possibly it messed up some setting or the authentication tokens for Siri servers.
Although I have performed OTA updates since they became available (with no ill effects whatsoever), I'm starting to wonder. The osxdaily.com post I referenced above states that:
After rebooting several times and trying a handful of troubleshooting solutions, I discovered this to be a fairly common issue with users who updated iOS through on-device OTA updates, though there doesn’t seem to be much explanation as to the cause.
I'm certainly clueless as to the inner workings of delta vs. full iOS (or Mac OS X, for that matter) updating, but... The download sizes of delta updates are obviously much smaller than those of the real McCoy (no offense to delta updates). Nonetheless, after performing a delta update via OTA, the iPhone behaves exactly as it used to after downloading a full-size new version of iOS via iTunes, as far as I recall. It's not a stretch of the imagination that perhaps Apple hasn't perfected the art of weaving the code it downloads from a delta update with the already-present code in the iPhone's current on-board iOS. JUST my $0.02!
That said, I think I may just perform "traditional" iTunes updates to my iPhone's iOS from now on. iOS isn't updated every week, and it's worth it for the peace of mind.