The only similar thread I could find was this one: http://www.zen-cart.com/forum/showthread.php?t=111395
Essentially, the problem is that when a guest visits the site and logs in, they simply return to the main page, still as a guest. If the member has items in their cart, they can log in just fine. Also, if the guest adds something to the cart and logs in during checkout, it works fine.
It seems like a session issue, I've run the Cache Key fixer, fiddled with permissions on my cache folder, and made sure my cart was fully updated. Anything else I can try?
Thanks guys.



