UPDATE: (Sep 6, 2007) Google just redid the entire A/B split testing process. See:

While GWO’s was originally created to test specific elements within a page, sometimes you want to test totally different versions of a page.

Lets say you want to test three totally different versions of page1.html. You create three new pages: page1a.html, page1b.html and page1c.html. You and then add some custom code to page1.html which redirects visitors to one of the new pages.

GWO added this feature a while back. You can read the official page at Comparing entire pages (A/B testing)

Recently, one of the posts on the Official GWO Forum mentioned a shortcoming of Google’s solution – the fact that it doesn’t carry forward the query string.

The Problem:
Lets say the URL of your main test page is landing.html and one of your test pages is landing_1.html.

If a user is sent to landing.html?code=123 they will be forwarded to landing_1.html and not landing_1.html?code=123 which is what you probably wanted.

The Solution:
This is easier then you might think.
Look for this line in Google’s original code for your test page:

document.location = test_url;

and change it to:

document.location = test_url + self.location.search;

That’s it!