We've seen a few minor outages for Siri since its debut, but those were minor blips compared to what appears to have been a more widespread, longer lasting outage that took place earlier today in the US, Europe and South Korea (according to Electric Pig -- we can attest to seeing an error message with the service in the US, and BGR noted it as well). We asked our Twitter followers and some reported brief outages last night, but others said it's been unavailable for hours and is still down.
Of course, when Siri was announced, Apple noted it was "in beta" but I took thi