BTW, be advised that not all buyers necessarily reach the thank-you page. PayPal Pro is the only payment processor that automatically refreshes to your thank-you page after successful checkout; regular PayPal standard and all other processors only provide a return/continue/finish link on their checkout-completed page, which link the buyer must choose to click manually in order to reach any thank-you page.
One way around this would be to enable the "Send payment data to a URL" setting for each product, then specify the URL of a script at your end. Once we receive notification from the processor that the buyer's payment was good and completed, we would POST order data to your specified URL, where your scripting could then invoke your Google AdWords/Analytics/Optimizer code automatically.
FWIW, for any checkouts done via Google Checkout, our integration with Google Analytics will report the sale conversion to Analytics even if the buyer does not click-through to the thank-you page, so Google may provide some way to integrate Analytics with Optimizer and take advantage of that, if you don't mind using Google Checkout as your sole payment processor.