Which magento versions are the certifications based on for both the plus and regular versions?
Are the 1.6 for regular and 1.12 for enterprise?
As of December 2013 it's Magento 1.7CE for Developer and Magento 1.12EE for DevPlus.
Source: http://www.magentocommerce.com/certification/developer
Pretty much the latest versions. Although the above answer is technically correct, you can expect challenge questions to include some stuff from whatever the latest release is.
Don't forget to study what a nominal product means...