I'm not sure that disabling it is a good idea, though. I've seen spam bots navigate successfully through some pretty complex use cases...I've had spambots register successfully on my sites that used activation! I had to use a captcha (ReCaptcha) to thwart them.
mad-q, although you are right about activation being a nuisance, I believe most people are used to it by now. In fact, I would venture to say that people, by now, would probably become suspicious of an e-com site that offers payments without account activation.
Just my opinion.