Stas - 2:16 pm on Jul 21, 2010 (gmt 0)
I think there has been some kind of sandbox for changes implemented very recently. So there's no point using expirience from months ago on this.
2-3 months ago I have been doing sitewide changes everyday, no page has ever dissapeared from google. As I've said earlier I had month and year in <h2> tag on every page of my website, I was changing it monthly, never a problem. Until I've changed it on 1st of july - Very product specific pages have dissapeared immediately. Still not coming back. I get few google hits to them from pages 160-200...
Pages coming back to first pages and dissapearing seems like the same sandbox effect as with new sites/domains.
Now this puts me off doing any on-page SEO. Maybe that's what Google wants? Or maybe it just thinks page headings (h tags) shouldn't really change - and sends it to sandbox.