Here's a Drupal 6 port. I'm calling it 6.x-1.0-beta1.

This includes the basic idea of the validation patch above, but is more extensive, and takes a different approach because of differences in the D6 FAPI. Any errors which would normally appear on the review page are instead shown on the checkout form page.

I have tested this successfully with the uc_credit module and a custom payment gateway module.