It is all tax free for officers too. It is on a month by month basis. If you spend 1 April in Iraq, all of April's pay is tax free.
It's not all tax free for officers.
Here is the link to the 2007 military pay chart.
http://www.dod.mil/dfas/militarypay/2006militarypaytables/2007_Web_Pay_Table.pdf
See note 4.
Officer pay is tax free up to 6642.6 which is the pay of the SGM of the army. So an LTC at max pay would pay tax on $700.00.