Mark,
When you say the receptacles are level with the bottom molding, I assume you mean that the bottom of the plate is level with the top of the baseboard.
I am far from an NEC expert, but I do not believe this is a violation of any article of the NEC. I can find no minimum height specifications after a quick scan through the most obvious articles which might be applicable (and keep in mind that even if it were a violation of the current NEC it may not have been when they were installed, so the installation would be "grandfathered").
If you haven't gotten a definite answer elsewhere, let me know and I'll check with some of my friends who are more familiar with the NEC.