Split testing

I’m interested in split testing my content to determine its effectiveness at generating conversions (in my case a download).

Split testing on AdWords is easy enough, but I want to split test versions of the same page so it works from organic search. For example, I have WP set up to render a static page as its front page. What I would like to see happen is when the front page is requested, we get a sort of…

<?php $testcase = rand(0,1);?>
<?php if ($testcase == 0) {?>
//Render page1
<?php };?>
<?php if ($testcase == 1) {?>
//Render page2
<?php };?>

There would probably need to be some cookie management so that a particular user gets the same version of the page for any 1 session, but it seems like a valuable thing to do.

Anyone seen this in action? Either via a plugin or hack?

Cheers