I've been thinking about setting a d20 modern game in my homebrew campaign world. Like many worlds, mine has distinct "ages," including a high-magic age, a steampunk age, and a modern age.
In the history of my world, the god of magic (who is the most powerful of the "good" gods) is killed by the goddess of death (one of the more powerful of the "bad" gods), which stops the flow of magic into the world. The resulting collapse of magical infrastructures users in an era of rapid technological developement, as the civilized races struggle to rebuild their societies. Additionally, any creatures that previously had supernatural powers end up being hunted to extinction, because they now lack their natural defenses.
The resulting age would look a lot like ours, but with elves, dwarves, and other non-human (but non-magical) races. As the new technological civilization evolves, the idea of magic becomes more and more fanciful, until the majority of the populace believes it never existed in the first place.
Here's the catch: outsiders can still use magic if they set foot in the mortal realm, and there is an invisible war going on behind the scenes where the servants of the good gods try to prevent the servants of the bad gods from taking over. This war is complicated by the lack of leadership of the now-deceased god of magic.
If I were to run a modern campaign in this age, the PCs would probably be caught up in the invisible war.