We know that virtual reality is being used as therapy to treat PTSD (as here, depicted above), but Doug Magyari, who's spent two decades working on VR simulations for US military agencies, raises a disturbing thought:

[V]irtual reality is a neutral technology, and like all technologies, it can be used for good or evil. In a VideoInk interview, Mike Rothenberg of Rothenberg Ventures suggests that terrorist groups might adopt virtual reality to unleash monstrous scenarios of torture and brutality. In effect, a virtual warzone would recreate the actual agony and bloodshed of war. And the lasting traumatic effects on the human memory could very well be similar to the real-life experience. We do not yet understand how these terrifying effects would be manifested, and that is why we should proceed with the utmost caution and foresight.

For that very reason, Magyari recommends that all VR content should "be made at a level of acceptability as if it was to be viewed by a 3- or 4-year-old child." (Which is quite a lot to ask, since pretty much any first-person shooter would be disqualified.) But I've talked with many combat veterans who've used Second Life as therapy for their PTSD with impressive results, and that's exactly what they're looking for -- not recreations of the horrific combat that cause the PTSD, but a placid, supportive virtual space where they can virtually escape from them:

Gwill and his fellow vets have created a space customized to serve their unique requirements-- which means isolating it from content you might typically see in SL or other virtual worlds. We keep our area at the center as a safe zone," said Gwill, "meaning we tell folks no weapons or gestures containing sound effects like that, or sirens... this way there is a place folks can go and sit down to talk and be safe."

In any case, the point Magyari raises is something I recently alluded to, when viewing a VR simulation of the 9/11 attacks: