Hey Donncha - great idea for a plugin!
I've just spent several hours trying to get it to work, but I'm getting 0 cached pages. Everything checks out - install is fine as far as I can tell.
No safe mode (PHP5/Linux/Apache/WP 2.31)
mod_mime, mod_rewrite, fancy permas on
.htaccess looks good (both)
WP_CACHE defined as true
I even made all dirs 777 for shits and grins - still no love
Have tried compression on/off to no avail. Compression not currently enabled in WP.
I am hitting pages with IE (not logged in), and another window in Firefox reloading the cache stats in wp-admin. 0 cached pages.