I put together a small tutorial explaning how to do this, which you can see here: **REMOVED BECAUSE URL DOES NOT EXIST ANYMORE**
(Sorry, but I'm not going to repost the entire thing here

The idea is that webshop administrators no longer have to approve all customers in a webshop, but at the same time maintain some level of safety, which is why a Captcha validation is used plus a one-time only link sent via e-mail to the customer. This ultimately means that customers can register and start shopping during the weekend, for instance, where webshop administrators may not have time to approve customers themselves.
I hope you will find this useful and hopefully the OpenCart team will consider integrating this in a newer version.
The modifications were performed in OpenCart version 1.5.1.3.1, but should work with all versions above 1.5.
Best regards
Kafoso