Supposing existing Checkout users convert at higher rates (I have no reason to doubt this -- existing holders of Paypal accounts do, too), there is one problem: there are so freaking few of them. Of the last one thousand people who checked out on my site, exactly _ONE_ had a Checkout account which was opened prior to the day they gave me money.

Guess where they opened that account? Yep. My site, the last time they had purchased from me.

[Edit: I feel Google should be paying me money for hooking so many people on their service, since apparently nobody else is doing the work. Come on, GOOG, "lifetype customer value" mean anything to you?]