I logged into the sitemaps page today and rather than seeing the verified tick for each site I had a link "verify" to click for each site to reverify the site, as well as a new link which said "Try our new meta tag verification method". Each site I have on the sitemaps system HAD been verified once already so why do I have to do it again? Of course I have done it again and it only took a few minutes, in fact the meta tag option was a lot quicker than uploading a blank file with a certain name as I can quickly edit my site template. Is this a frequent or reoccuring thing with sitemaps? I must be honest, i have only been using sitemaps for about six or seven months so perhaps this happens twice a year for all I know. I'm a tad confused.
Hmmm...I guess that I am not the only one then. That is the second time that has happened to me this year as well. Maybe everytime they roll out new features or updates, you have to re-verify? I dunno. Please let me know if you find something out. Thanks!
On another site I found the following: "Do not remove this file from your Web site. Google will periodically check to make sure that this file still exists. If it is removed, Google will ask that you re-verify site ownership" It could be that during a time when Google were re-downloading the sitemaps from my server that the server was periodically down and hence they had to get it re-verified. Just a wild stab in the dark, I really do not know for sure. Hopefully I won't have to do this often!
Usually, if you have added new pages to your site, you should re-generate a new sitemap so that google will index all pages in your site. This could happen when google crawls your sitemap and found that the size is different then they will request you to re-verify.
I posted some information in this blog post a few days ago: http://sitemaps.blogspot.com/2006/08/time-to-verify.html We'll be checking verification more frequently now, but if you leave the verification file or meta tag in place, your account will remain verified.