Alternatively, does code.jquery.com offer any kind of uptime guarantee? Big companies would rather rely on their own hosting / CDN than that of jquery, I'm sure.
Even for a small company/project, I've stopped using external CDN.
The gain is not significant enough to balance problems like this one. It's not often than a consumer ISP does this mistake, but that's pretty common for a user behind a corporate proxy.
You link to Google's JS so your users can still get it when your own site is down?
If you hosted it yourself your users would see a working site whenever you were up. Now they will see a working site whenever both you and Google are up. This has not improved effective uptime for anyone. (On the other hand, Google might serve the JS faster, especially if your users cache it. They may also keep their copies more up to date.)
The gain is not significant enough to balance problems like this one. It's not often than a consumer ISP does this mistake, but that's pretty common for a user behind a corporate proxy.