Undead are immune to [Death] effects, but the Assassin's Death Attack is not a [Death] effect.
It is contingent upon a successful sneak attack though, which Undead are immune to as a result of being immune to critical hits.
Even if you somehow (feats, spells, the new augment crystal in the forthcoming Magic Item Compendium, etc) manage to Sneak Attack an Undead, the Death Attack *still* won't work because Undead are immune to any effect that allows a Fort save unless that effect can affect objects (i.e. Disintegrate, which works great against undead with their typically poor Fort saves)