I'll mention what I mentioned about a year ago in response to a post on our forums about "Is WoW evil?"
D&D is evil as far as it:
1. Interferes with your Christian walk -- ANYTHING is evil if it interferes with your Christian walk.
2. Interferes with your family -- see above note.
3. Passes falsehood and untruth into your way of thinking -- This is a hard one, but it is really really important, specially when it comes to D&D. So, as I don't really play D&D (my mom would KILL me) and I don't wanna sound like a know-it-all talking about what I don't know, I'll talk about what I DO know: WoW. In WoW, much of the lore and history is based on (specific names here) Animism, Panatheism, and Paganism (I may have the names wrong so I will specify: Belief in Elemental Spirits, Belief in Over-Arching World Spirits, and Belief in Animal & Plant Spirits.) Oh, and you could throw in the non-functional impersonal "Light" belief.
I still play WoW, but I am careful to notice and catalog as untrue all of the "Belief in Spirits" stuff that WoW has in it. I don't ignore it, but instead I avoid allowing it to impact my way of thinking/worldview.
4. Goes against what your concience says -- Your concience, if you are a believer, should be attuned to the will of the Holy Spirit, and therefore should be a guidepost telling you what is bad or good for you in light of God's plans for you. If you are worried that your concience could just be a "guilt-reaction" caused by years of bad teaching on the subject of D&D, pray and ask God to clarify for you whether it is Him speaking or just an ingrained reaction.
5. Interferes with the spiritual lives of those around you -- This is what Paul is talking about when he talks about "Meat offered to Idols." The meat was not in itself evil in any way, but it was causing serious stumblage to those around the Christians who had no problem eating it. If your playing of D&D would cause major issue in your family or church, avoid it!
<<<And now a brief note: If you haven't had issue playing games like "Neverwinter Nights" or "Baldur's Gate" bear in mind that there are ports of D&D... if the Holy Spirit hasn't warned you about those, chances are that if you plug into a good D&D group, then you shouldn't have much of a problem>>>
<<Oh, and there are many good Christian D&D groups out there>>
<and many bad Christian D&D groups>