All this is silly.
Critical hits are clearly misnamed - that's why everyone is debating this. The intent of critical hits is not to say "Sweet, I chopped his brain in half with my x2 damage!". Rather, critical hits mean "Sweet, I hit him really hard that time; that's going to leave a mark!". So they're not really "critical" at all. Just better. But calling them "better hits" sounds pretty lame, so they went with the cool name that misleads everyone.
If critical hits were actually "critical", then they would be literally damging/destroying vital organs. An average guy hitting you with a longsword that scores a critical hit would do 9 damage. If you're a 3rd level fighter, you can recover that HP loss in 3 days of good sleep at night and light activity during the day.
I would hardly call that "critical". I have had deep paper cuts and mosquito bites that took 2 or 3 real life days to heal.
By that definition, critical hits are just doing extra damage because you landed a solid blow that your opponent didn't handle well.
So, arguably, they should apply to just about everything.
On the other hand, it's equally arguable that certain types of creatures are immune to these "solid blows".
For example, a skeleton or zombie might be immune because he can't lose extra blood, can't suffer nerve damage, and feels no pain or shock from losing extra tissues.
In my mind, I see battles with skeletons resulting in bones flying everywhere. You hit a skeleton in the chest with a sword, and ribs are broken. With no skin or muscle to hold the broken ribs in place, they just fall off (or fly off from the impact).
Sure, maybe breaking a few more could destroy a skeleton faster, so a "solid blow" critical should destroy more ribs.
Or maybe losing those extra ribs from a critical hit means nothing to a skeleton, thus it's really immune since it doesn't destroy it faster.
That argument could go either way.
They way I look at it is this: When you're fighting a skeleton, you are attacking it with everything you've got. You slam your sword into it as hard as you can, every swing. Sometimes your aim is off (you roll low damage), other times your aim is good or great (you roll high or max damage). But you cannot crit it because your max damage is all your weapon can do to that skeleton - it cannot sustain shock or trauma or bloodloss or nerve damage or organ damage or any of the other stuff that makes criticals ("solid blows") effective against living creatures.
And rogues absolutely cannot sneak attack anything that lacks vital targets: organs specifically. Joints and limbs and necks don't count as vital targets since there are no rules in sneak attack for disabling someone's elbow, chopping off their hand, or decapitating them, etc.
YMMV.
As for the idea that critical hits could represent chopping off a skeleton's head, there is no reason to assume that losing its head would bother it in any way (other than the loss of some structure representing the HP that it lost). It can still make its slam attacks. Same holds true for golems, zombies, etc.
And as for chopping off an arm or a leg, well, yes, that would reduce its ability to hurt you, or chase you. But the same is true of lopping off the arm or leg of an orc, or a dragon. However, there are no rules presented for lopping off the limbs of living creatures, so using such a notion to justify critical hits against undead or constructs is pretty silly - apply it universally or don't apply it at all. It will be pretty embarrassing when you're DMing a group and your fighter starts arguing that his critical hit just chopped off the evil wizard's arm because he rolled the same damage as the critical hit that chopped off the skeleton's arm last week.
Admittedly, a few immune creatures seem to be exceptions. Vampires are pretty high on the list of creatures that might be an exception. Still, I don't think you will get any mileage from jabbing a vampire in the kidney, and said vampire is still immune to bloodloss (he needs blood to live, but he doesn't have a beating heart so his arteries don't spray blood like the living), nerve damage, trauma, or any of the other fun stuff that critical hits ("solid blows") might represent. And staking or decapitating really sounds like coup-de-grace attacks given our current ruleset.
So as for me, I like the way 3.5 handles this stuff, I still enjoy playing rogues (I get undead bane weapons so that I can still participate in those fights), and I hope 4e continues with much the same type of differentiation.