So, like a [10th Kingdom][1] thing, where fantasy characters are running around a modern setting? Or more of a Top Cow Lady Pendragon thing, where fantasy characters are reincarnated in a modern setting?

Assuming the former, since they're both d20 system, I wouldn't bother with an explicit convert for the PCs. Have the PCs continue playing with standard issue DnD3.5 rules; convert the environment instead. For example, you might want to increase the damage on firearms.

The only thing you may need to convert for the characters themselves is how to give the players a "wealth bonus" since the DnD gold standard doesn't really work in a modern setting.

(Community Wiki as @C.Ross kind of said this first, in [comment form][2])


  [1]: http://en.wikipedia.org/wiki/The_10th_Kingdom
  [2]: http://rpg.stackexchange.com/questions/6180/converting-dd-3-5-characters-to-d20-modern-characters#comment-7554