I'm sure plenty of you fellow atheists out there get pretty annoyed whenever some fundie claims that we have no basis for our morality. I know my old blog partner feels that way, and I get pretty foamy myself. So, I feel like posting something to hopefully convince the fundies to get the plank out of their own eyes before claiming there's a speck in ours.
The fundie claim usually stems from divine command theory and ignorance of other ethical theories out there. One dilemma I have yet to see a reasonable answer for is the Euthyphro dilemma: Do the commands of the gods define morality, or do the gods command what is already defined as moral? Let's take a look at all the answers I've seen thus far (Please note that this will focus largely on Christian fundies, since I haven't had as much experience with others. I'm sure they've got their own equivalent nastiness):
Possibility #1: God defines and redefines morality whenever he wants.
This is probably one of the worst scenarios: First, many fundies out there interpret this as 'anything goes': They can claim whatever they do is "God's will" and thus, the alleged moral high ground.
One really nasty example I'm aware of: The Christian stance on human sacrifice: First, God commands Abraham to sacrifice his son, and steps in at the last second to tell him the whole thing was a test of his obedience. Then, in Judges 11:29-39, assuming God is omniscient, goes through a roundabout method of commanding Jephthah to sacrifice his daughter. I don't think I need to get into all the nastiness about whole Jesus story. Yet today, most Christians would probably say, without equivocation, that human sacrifice is wrong, relying on apologetics to rationalize away the pro-sacrifice portions of the Bible.
Under this system, the phrase "God is good" is essentially meaningless: "Good" changes according to whatever capricious whims God has. It's also impossible for us mortals to determine good and evil: Without a direct, real time feed from God's mind, we can't know, since he might define not informing us of changes to be good.
Possibility #2: God defined morality only at the beginning of the universe and kept it the same.
If this is the case, there's a lot of explaining to be done:
Possibility #2a: God defined certain things as evil, and chose to do them anyway. God seems like a rather nasty character to me. He isn't afraid to let innocent bystanders get in the way of his personal "glory," dabble in genocide, or promote slavery. Most Christians (I would hope) will not hesitate to call these things evil. This leads me to wonder why they would respect, much less worship, such a being.
Possibility #2b: God defined morality in a consistent manner that allows him to perform all the items in 2a and still be "good."
If this is the case, someone should be able to specify what conditions and circumstances allowed those acts. If it's beyond human comprehension, then morality is a bureaucratic, circumstantial nightmare we can't navigate without a supreme being. The end effect is no different than possibility #1: Anyone can say that God gave them the go ahead, and we can't judge them.
Possibility #3: God defines morality differently for humans and for himself.
Why the double-standard? The general vibe I get from this scenario is that might makes right: God's all-powerful, so he can do whatever he wants. Not much different than 2a, is it?
Of course, all of the above suffer from the same central problem: What enables God to define morality? What specific characteristics give him that ability, and where'd the relevant rule come from?
Possibility #4: Morality defines God. For whatever reason, God is inherently constrained by preexisting moral law.
If this is how morality and God work, what's the basis for claiming that atheists have no basis for morality? We just don't believe in the middleman. Of course, the problem with this is that the Biblical God is still a rather nasty character as written, unless someone would like to argue that slavery, human sacrifice, and genocide are morally permissible.
There are probably plenty more possibilities, but I don't currently see any reason to elaborate. I will if someone brings them into relevance.