As far as the mindless undead are concerned: Yes. At least to a certain point. Then again, in my campaign they're occasionally encountered as natural phenomena - skeletons arise from unburied/unavenged 'sleep' with worrying regularity. This largely involved them being found in a sealed room covered with dust, however; the decay would probably be slowed by this. They don't last past one activation most of the time.
For intelligent undead, I figure they're being transformed into something else by the negative energy spirit that gives them purpose beyond death. (Then again, not everyone goes with that interpretation.) If you accept that, then you'll accept that they retain certain cell processes, but their cells operate in a different way - slower, no heat, reproducing only occasionally to heal wounds or replace mortified tissue, in which case the undead slowly approaches a different physical state over centuries. (See my game page, linked off my comic below, for one interpretation of this as it applies to ghouls.) Because they have no real immune system or body temperature to fight off infection, they may still rot (especially ghouls), but they won't fall to pieces because they'll heal magically. This allows for centipedes to crawl out of a ghoul's nostril at a dramatic point - it's a nice dark place to nest, really, isn't it?
Incorporeals are, of course, immune to any decay except residual self-image. That is to say, they won't rot unless they think they should. Then again, incorporeals are usually insane anyway, so who knows what they think?
There is one exceptional encounter later in my campaign involving underwater zombies; I guess they're preserved by the negative energy or something. (And when I get around to chronicling it, you'll understand why.)
Really, it's all up to taste and circumstance, isn't it?