NOTE: this may not apply to the most recent versions of the plugin
Just to add a warning. We have a WP 2.0.5 site with an earlier version of the plugin installed. We found that Google was not indexing our site. After a lot of research and investigation, it turns out that the problem was caused by WP-PostRatings. I think we are using version 1.02.
It seems there is some code in the plugin that checks if the user agent is Googlebot, and if so then it just exits the PHP script for the page. Hence Google will never be able to see any of your site. It's a pretty nasty bug, and difficult to diagnose. For more details see this thread: http://groups.google.com/group/Google_Webmaster_Help-Tools/browse_thread/thread/1b6b8f918e3a58cf)
Hopefully this problem has been fixed in the more recent versions of the plugin, but I thought I would post our experience here just in case someone else runs into similar issues.