I'm getting reports from a client that PCs running IE8 & 10 on an internal network with ChromeFrame plugin installed won't stay logged in to a site that is set up with all the content behind a login form, i.e. on the front-end, not logging into the control panel. 
Disabling the plugin in the browser makes the problem go away. 
So does updating .htacess from 
<IfModule mod_headers.c>
  Header set X-UA-Compatible "IE=edge,chrome=1"
  <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>
to 
<IfModule mod_headers.c>
    Header unset X-UA-Compatible
</IfModule>
This seems to have solved the issue but I thought it was worth mentioning here anyway.