Google sites removes the dot in the verification meta-tag of the Bing Webmaster Tools. Luckily, there is a workaround.
When registering your Google Sites site in Microsoft Bing Webmaster Tools, you get 3 options to proof your ownership of the site.
The easiest way is to choose for option 2 with the <meta>-tag.
The meta tag is similar to this:
<meta name="msvalidate.01" content="xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" />
Bing asks you to add the <meta>-tag to the <head> section of your website.
In your Google Sites site, go to the site settings (Manage Site) and click General on the left column. One of the settings is Google Webmaster Tools.
Copy paste the <meta>-tag above to the Google Webmaster Tools setting.
When you click Save, and inspect the source of your Google Sites site, you will find the <meta>-tag, but for some reason, the name is changed from
msvalidate.01 to msvalidate01 (without the dot). Consequently, Bing Webmaster Tools tells that the meta-tag is missing, and they could net verify your ownership of the site.
A workaround for this problem is to use msvalidate-01 (with a hyphen) instead of msvalidate.01.
The hyphen is correctly displayed in the source of the website, and Bing Webmaster Tools succesfully validates your ownership of the site.