d20 Apocalypse just came out, WotC's quiet nod to RIFTS d20 players (among other things), with some neat stuff for a RIFTS type game using d20 Modern (which is the core engine I've seen used most often, over normal d20).
The d20/Rifts conversions I have seen have worked roughly like this (and I've seen two different campaigns by two different GM's with independently created conversions):
Start with d20 Modern, use Urban Arcana for some of the technology rules and some of the expanded magic rules and low-tech/fantasy crossover stuff. d20 Future is your friend with regard to mutations, mecha/power armor, and uber-weapons. d20 Apocalypse appears to have a fair amount of useful stuff too. The Menace Manual has a lot of creatures and some equipment that would probably be useful (including interdimensional stuff).
A lot of Rifts staples are things available in d20 Modern/Future with different flavor text and roleplaying. Mechanically, a Juicer could have the Maniac template (d20 Menace Manual) and the Dreadnought and Helix Warrior advanced classes (d20 Future). A Glitter Boy suit is just normal mecha with a big gun and a special Damage Reduction against laser weapons, Crazies can be simultated with the mutations and cybernetics systems.
I've known of some people who have written up custom advanced and prestige classes, and conversions of some races ect, but I think Kevin Sembieda would launch his Tactical Nuclear Lawyers if I posted them, but it's been done.