Retaining Cart Contents after Login

Surely I'm not the only one who's had this issue.

A customer shops on our site. They build a cart while not logged in - items show up in our custom SSP cart, which is on our primary (non-secure) domain. When hitting the checkout button, the page sniffs out that they are not logged in, and forwards them to the registration page. They are now on the secure subdomain  (registration SSP page) and the state of their cart is lost to Netsuite. On the back end of my register.ssp render, order.getItems() produces nothing.

If they log in as an existing customer, Netsuite will find the last shopping cart they had active and either merge it with their present items or replace it altogether. This is a bad thing while in the middle of the checkout process, as the next thing they will see is the final checkout page.