Tell one player you're running a d20 Modern game and have her make up a character. Ther rest of the players know the truth though, you're actually running D&D and all their characters are made accordingly. Upon start of the game, d20 Mod character is confronted with a demon, Giant Evil, whatnot and is hurled into a portal/unconsciousness/whatever. PC wakes up in a medieval D&D world of your campaign, and joins the other PCs in their quest, largely with hooks at finding the boogie monster that brought him here in the first place.
Now immediately, the unprepared player has likely made some choices that might seem worthless now. Perhaps if the PC took many ranks in Computer Use, it could be found that the PC's knowledge of programming is analogous to magic, and could make spellcraft checks with the skill (maybe be able to get a bonus to Use Magic Device). Repair could likely be used to restore HP to constructs and objects. Craft might be able to build modern items in the D&D setting(using d20 Modern craft rules) a la Ash.
The Modern PC could advance in any Modern basic class, as well as any D&D class she chooses. Only Modern classes are a favored class to the Modern human. Modern Advanced Classes are not available. The PC will have no action points, unless you're playing in Eberron.
Certain consideration arises if the Modern PC is created as a gamer. Should the PC manage to bring D&D rulebooks into the world, certain things change about them:
~the PHB become a spellbook with mildly useful (though unfamiliar, must be deciphered) spells within.
~the DMG is blank, no cheating
~the MM gives a +5 bonus to the appropriate knowledge check regarding indentifying and understanding a monster
~Any regional FR book (assuming you're playing in FR) or the Greyhawk Gazetteer (assuming you're in Greyhawk) Gives +5 to Knowledge (geography) or Knowledge (local) if appropriate for the book.
Though without prior knowledge of the game's purpose, I don't see this occuring much.
