Please standy, as jjdodger and I are, currently, discussing the DZ for Infiltrators - apparently, I am not entirely correct. Answer forthcoming.
Okay, Jeff has pointed out my errors, and has cleared this up, a bit.
When deploying the infiltrating Unit Card, it may not start closer then 12" to any enemy models or cards (in the case of cards, measure from the center of the unit card/marker/whatever) nor may they start in the enemy Deployment Zone.
If the enemy cards or models move, the area in which to deploy the infiltrating models does not. Their DZ was defined by where the enemy models were imediately after Deployment, but before the first activation. When the models are deployed, the must be within command distance, and may not start closer than 12" to the spot where any enemy models are, or were, at the beginning of the turn, nor may they be deployed into the enemy DZ. Use markers for enemy models that move away from the spot that defined the Infiltrators DZ.
If enemy models move closer than 12" to an infiltrator, whether they Spot the models or not, does not change the original Infiltrators DZ. Just because the enemy moved closer does not mean that the infiltrators have to back off. They are still entitled to deploy within command distance as long as it is no closer than 12" to the spot where any enemy models are, or were, at the beginning of the turn, nor may they be deployed into the enemy DZ.
Does this clear it up?