This plugin helps you to integrate Google services (Analytics, Webmaster Tools, etc.) with Your Blog.
Starting from October 2009 Google checks contents of file used for verification - it should contains something like this:
Please make sure that you use Google Integration Toolkit in version 1.3 or newer. If you do, open verification file in your web browser and check page source to make sure there is no extra content (e.g. HTML comment). I am aware of WP Super Cache plugin, which adds such comment in version 0.9.6.1 or older - if you use it, please upgrade to version 0.9.7 or newer. If you find another conflicting plugin, please inform its author about this issue, or me.
Starting from October 2009 Google changed name of its meta verification tag from
<meta name="verify-v1" content="..." /> to
<meta name="google-site-verification" content="..." />. Please make sure that you are using Google Integration Toolkit in version 1.3 or newer, and check its configuration to make sure you use the new tag.