The only source I‘ve found that gives a hint here is the Revive Undead Spell (Spell Compendium, 175). According to this spell an undead "destroyed by hit point loss" is an inanimate corpse rather than a pile of dust.
Your restore animation to an undead creature destroyed by hit point
loss (even a zombie or skeleton, which can’t normally be reanimated
once destroyed). You can revive a destroyed undead creature that has
been inactive for up to one day per caster level. (...) The body of the undead to be revived must
be whole. Otherwise, missing parts are still missing when the creature
is reanimated. None of the dead creature’s equipment or possessions
are affected in any way by this spell. An undead that has been
destroyed by a turning effect can’t be revived by this spell.
It's important to note that it makes a difference whether an undead is destroyed by HP damage or by a turnig effect.
As Yopi Lapi pointed out that the original version of this spell (Libris Mortis, p 70) has a different wording which reads: "An undead that has been turned to
dust by a turning effect can’t be revived
by this spell (because only dust remains
of the undead)." (emphasis mine)
While this line makes the difference between a turned and a slain undead quite clear, it was - for some reason - revised in the Spell Compendium - to leave things in the dark again.
Anyway: if not turned, the Curst should be able to "fool" the party...