Would a party composed of PCs with Darkvision and no light source see an undead shadow in a dungeon? How would this translate to game terms?
If you start thinking about it, you might end up with a headache. For simplicity reasons: just take the standard rules:
From the SRD
A shadow gains a +4 racial bonus on Hide checks in areas of shadowy illumination. In brightly lit areas, it takes a -4 penalty on Hide checks.
Roll Hide (+12; I would still rule it is shadowy illumination, although by the RAW it is not for people with darkvision) and have the PCs make a spot check to see if they notice the shadow.
If you wanted to be a punk, remember that darkvision is "just like normal vision, except black and white".
So, a smart dungeon overlord might paint the rooms of his shadow guards black. This might give the shadows "partial concealment" in normal light, and even "total concealment" in darkvision. Permanently invisible shadows? Sounds like fun to me!