Good news for Rifts converters:
d20 Hell of Earth should be out soon. The game features a lot of similar themes (except aliens). From personal experience in d20 Deadlands, one can easily use the game mechanics and take or leave the "it's not really tech, it's just a new window dressing for magic."
As an aside, my personal take on d20 conversions is not to "teach the guy off the street." DND is still a little too daunting for that ("THAT'S the rule book!! It looks like a novel!!) d20's real strength is in getting players too lazy (or too comfortable) to learn a new system. A lot of the "innovations" in 3rd have been sitting on my bookshelf for years in other games -- it just didn't have the brand name attached to it.