Glad to say I have finally found out why a few people were always getting the 'Key Has Expired' when using Ecaptcha.
Turns out Microsoft IIS webserver does not support the $_SERVER['REQUEST_URI'] variable that ecaptcha was using, so I have had to use a close alternative of $_SERVER[SCRIPT_NAME]."?".$_SERVER[QUERY_STRING] for people not running Apache.
I have added a fix for this, so ECaptcha 1.6 is now available for download.