jojo mentioned this briefly in his balance thread from many months ago. A player should be able to evacuate an immobilized unit.
The reasoning is quite simple: In sealed, almost everyone would grab any immobilize debuff before grabbing an evacuate, and I think most people would agree that immobilize is a better card. Therefore, allowing a player to evacuate an immobilized unit would make evacuate slightly better and immobilize slightly worse, hence acting to balance the two cards.
However, there is the issue of an evac being played on the same turn as an immobilization. There will be, to my knowledge, a 50-50 chance of which gets played first.
Scenario One: The evac gets played first, the unit is evacuated, the immobilize is not played. I believe this is possible currently.
Scenario Two: The immobilize gets played first. Currently the evac will not work if the immobilize wins the coin flip. Are there any concerns of allowing an evac to be played after losing this coin-flip? Would the evacuater be upset that he evacuated an immobilized unit? Probably not because immobilized units usually get killed, and evacuation is usually used to save a troop. Would the immobilizer be upset that he used his card on a unit that was evacuated? Possibly, but he still succeeded in taking that unit out of the battle for two turns.
I don't think we need to be concerned that a player who used immobilize will be upset that it was used on an evacuated unit. The main reason is that the player who used evacuate, if using it in anticipation of an immobilize, had to guess correctly or risk wasting his evacuate.
The point of evacuate is to save a unit in trouble by returning him to your base. What unit could be more in trouble than an immobilized unit? Evacuate isn't a top notch card while immobilize is, so it should at the very least be allowed to counter-act an immobilize by returning a unit to your base.