Well, we've had an update on the adsense forum at Google. They admit the rate was wrong, are working to resolve the issue and will contact impacted publishers via email soon.
A slightly better start to the day than yesterday!
@piatkow, my comment was a little bit tongue in cheek, as yes a human probably does input the rate at some stage. But you would have thought that there would be checks in place to prevent this kind of thing happening especially where the data entry affects outgoing payments? Just a simple alert when the figure is say plus or minus 10% of last month with override, or a second set of eyeballs on the figures. I realise none of these are foolproof but little cost to Google.