grep for the 404 message in your access_log files, it will indicate whether the RewriteRule matched or not.
grep " 404 " access_log
either you will see
127.0.0.1 - - [11/May/2012:23:50:56 +0100] "GET /index.fcgi/XXXX HTTP/1.1" 404 123
or
127.0.0.1 - - [11/May/2012:23:50:56 +0100] "GET /XXXX HTTP/1.1" 404 123
the second ones indicates that the rewrite didn't match...