Our friend Shawn Merdinger dropped a note pointing to this article in Federal Computer Week that indicates that the US Army is now using VoIP in the field:

Not long ago, if an Army infantry battalion commander wanted to communicate, he relied on radios that had limited range and often spotty coverage. But today, a growing number of those commanders can use a phone connected to a Joint Network Node terminal and call anyone worldwide.

The Army has harnessed the power of voice-over-IP telephony for use as a tactical, battlefield communications system with JNN, said Jim Sintic, technical director of the Army’s Program Manager for Tactical Radio Communications.

The article has more information and a picture as well. No direct mention of the security of the calls (although there is a bullet point in a sidebar indicating the JNN provides a secure Internet connection), but I would have to assume that all is being encrypted. (Anyone with more information is welcome to leave a comment here.)