It isn't obvious to me what I should troubleshoot next - is this a session issue?
How To Reproduce:
Add 5 items to cart (I'm testing with products over $100)
Proceed to checkout
Proceed past billing -- throws 403, redirects to cart view.
I do see the code below in Mage_Checkout_Onepagecontroller. If anyone has any insight into why the session might be expiring during checkout, if it is at all, I'd love to hear it. In the meantime, I'll report back with any other findings.