Update #4 (13:17 Zulu): There appear to be lingering issues with posting/editing at least, possibly other areas, site is offline again.
Update #3 (12:58 Zulu): Looks like SO is back online: https://stackoverflow.com/
Update #2 (10:54 Zulu): Getting the SO error page now, she breathes!
Update #1 (10:50 Zulu): Confirmed on twitter by @Geoff Dalgas:
StackOverflow is down - ISP power issues - we will be back up soon
Remember that meta was migrated to NYC recently, so it's still up, looks like everything in the old datacenter is offline at the moment, for whatever reason :)
- stackoverflow.com - 126.96.36.199
- serverfault.com - 188.8.131.52
- superuser.com - 184.108.40.206
These (and most of the other sites) are all being routed through the HA Proxy in the old datacenter.
- meta.stackoverflow.com - 220.127.116.11
- area51.stackexchange.com - 18.104.22.168
These sites are in the NYC (new) datacenter, and still online.