darkmage - 11:48 am on Mar 8, 2007 (gmt 0)

"Overnight, average EPC jumped 500%. Impressions and clicks remained the same, only the amount I was earning per click changed. Now it could be argued that changing the publisher ID forced Googlebot to re-look at the content of the pages, but this was an established site and the advertisers did not change noticeably."

Googlebot is the exactly case - it is the same effect as deleting or changing a channel. My experience is that Google resets a page when a key parameter* changes. It takes a little time for 'correctly' targeted ads to return. Getting back to the thread, this can create substantial variance in the ads and CPC - sometimes good, sometimes bad.

*Key paramater - content update, same URL (Googlebot is slow to change the ads)
New URL - instant change (even if same content)
New Pub ID, Channel, layout type - Google can detect this straight away