you can write a story taking place in the same universe as another story as long as you have permission. I know your not writing a book in the dresden universe, but say you did, it was good, and you wanted to sell it, you could go to the dresden publisher, or butcher, whoever owns the rights basically, and see if they would publish it.
many publishers use established universes to try out new writers, and get their names known. that's why you see so many star wars, star trek, dragonlance, warhammer books out there.
as for copywrite infringment, legally speaking, the holder of the copywrite has to proove that you stole their ideas. and as before mentioned it is very hard to proove that an idea is stolen, not just similar. plus if the holder of the copywrite borrowed their themes and elmemnts, it is much harder to proove. for example, how many books have we all read that have evil wizards and orcs verses an alliance of elves dwarves and men? I have personnally read like five trilogys that copy the lord of the rings, almost to the point of story arc, and they didn't get sued. just because the similarities are all classic archtypes.
so its all about the points of similarities between two works. if you leave characters and specific names out of a dresden copy, you just have magic hiding in the real world, and you are in as much danger of getting sued by the owners of buffy or harry potter as you are of dresden.