bluemamba - 9:35 am on Oct 27, 2010 (gmt 0)
I'm seeing reports of this phenomenon over at the Adwords support forum as well. It's too early to tell but, based on the available evidence so far it's either: 1. A massive bug or 2. A huge quality score slap.
If I was a betting man I'd wager 65% chance of the former and 35% of the latter. Although Google has improved a bit in recent times by pre-announcing modifications to their QS algorithm, they have a proven track record of releasing the biggest, most far-reaching changes without any notice at all.
When they first launched QS (in 2006 if I recall correctly) they unchained the beast with no warning and let it reign its destruction far and wide without so much as a courtesy email.
Not impressed - in either scenario.