I imagine that one would usted anyone outranking them, but does this politeness extend to addressing subordinates?
My instinct as well is that when speaking to someone of the same rank, one might use usted for formal communication and tú for informal.
Does it vary from country to country?
En España está estipulado en el BOE (Boletín Oficial del Estado).
Art. 462. Todo militar que deba dirigirse de palabra a un superior se cuadrará ante él, saludará y le dirá: «a la orden de (tratamiento) mi (empleo del superior)», cuando tenga tratamiento de Excelencia o Señoría, y «a sus órdenes mi (empleo del superior)» cuando tenga el de Usted. En la Armada, cuando corresponda, se dirá «mi comandante», «mi segundo», «mi tercero» o «mi Oficial».
Art. 491. Los Reyes de España tienen el tratamiento de Majestad; el Príncipe de Asturias y los Infantes de España, el de Alteza Real, el Presidente del Gobierno, los Vicepresidentes si los hubiere, el Ministro de Defensa y los Oficiales Generales el de Excelencia; los coroneles y capitanes de navío, el de Señoría, y los restantes miembros de las Fuerzas Armadas, el de Usted. Reglamentariamente se determinarán las distintas formas de expresión oral y escrita de estos tratamientos.
Fuente BOE
The official answer for Spain would be the one provided by Danielillo. Nevertheless, this is not what happens in day to day life. In informal communication, most officers tend to use tú when addressing subordinates. But there are exceptions. For instance, in the Navy it is customary for officers to use usted and even Don when addressing high ranking NCOs.
I once met a Navy Commander who always addressed his assigned Warrant officer as Don José. He explained me that this use was based on the roles in a ship (they were both actually working at the High Command, on land) and many Navy people liked keeping this tradition.
Age also comes into consideration. Usually, young officers don't feel confortable addressing veteran subordinates as tú and may resort to usted.
In summary: subordinates almost always address their superiors as the ordnance says, but superiors tend to use tú when addressing subordinates, with some exceptions.
