It looks like the plugin you’re using inserts the following meta tag:
<meta http-equiv="X-UA-Compatible" content="IE=9">
I’ve read about folks saying it doesn’t work as good as putting it in .htaccess.
Here’s what I copied out of HTML5 Boilerplate’s .htaccess:
# ----------------------------------------------------------------------
# Better website experience for IE users
# ----------------------------------------------------------------------
# Force the latest IE version, in various cases when it may fall back to IE7 mode
# github.com/rails/rails/commit/123eb25#commitcomment-118920
# Use ChromeFrame if it's installed for a better experience for the poor IE folk
<IfModule mod_headers.c>
Header set X-UA-Compatible "IE=Edge,chrome=1"
# mod_headers can't match by content-type, but we don't want to send this header on *everything*...
<FilesMatch "\.(appcache|crx|css|eot|gif|htc|ico|jpe?g|js|m4a|m4v|manifest|mp4|oex|oga|ogg|ogv|otf|pdf|png|safariextz|svg|svgz|ttf|vcf|webm|webp|woff|xml|xpi)$">
Header unset X-UA-Compatible
</FilesMatch>
</IfModule>
You may want to disable that plugin, and stick the above in your .htaccess (above your wordpress and s2member) mod_rewrite rules.