Also note that Detect Evil will detect clerics of an evil deity, even if the cleric's alignment is non-evil, and will detect undead, even if the undead's alignment is non-evil (or even good).
What is unclear is if a Chaotic Evil vampire, for example, will be detected at all by a Detect Chaos spell. It's an undead creature, but the spell is "as Detect Evil, except...", which means that it is not considered a Chaotic creature for purposes of the spell, but rather "Undead". However, Detect Chaos does not detect "Undead" specifically... so the spell can be read to say that Chaotic undead are not detected by Detect Chaos. (Substitute "Law" or "Good" in the same argument.)
-Hyp.