Tieflings = Melniboneans?

Well, not exactly. Melniboneans had their pacts with the gods of Chaos. D&D 4th edition tieflings, apparently, had their pacts with devils which, in older versions of D&D, were lawful evil. This comes from ENWorld's news of December 9th:

Tieflings are not human and demonic offspring, but are the true-breeding descendants of an ancient empire that made dark and terrible pacts with the Nine Hells. Their fiendish visage is actually a manifestation of a curse, due to their progenitors' crimes. They're more closely tried to devils than demons.

I'd love to give you a direct link, but I'm not seeing how to do that just now. :p If you can find it, there's lots of neat stuff from the "Races and Classes" preview book for 4th edition D&D.