Google website optimizer is not specifically for g checkout, adwords, or other G services - you do need an adwords/analytics account because it involves the same tracking code - but you dont have to use these accounts.
It can be used for anyone that wants to do split test/multi variant testing on a given web page.
I signed up about a week ago and am very into what Im experiencing - I'm currently doing a multi variant test on a sales page - swapping out the main text, the way the products are displayed, and the way the price of the products are displayed. It's been about a day since I started the test and Im seeing the results in the reports: it shows me how each variant performed against the original, and which combo of the three variation tests within the test page are performing well.
Performance is measured by sales for my test, I indicated a successful conversion page. It can be used to test signups or other website visitor actions.
I'm sort of a geek about these things, and if you are too or just interested, I started a simple blog tracking my progress and explaining the problems and stumbling points I ran into - you can read it here:
Goptimizer (thats short for Google Optimizer!)
meegwell