Johnny-Dee has an interesting criticism of the doctrine of divine simplicity at FQI; naturally, I disagree with it, as you can see in the comments, but it's interesting. John always has a knack for putting things in a clear and straightforward way. [UPDATE: John has a follow-up post.]
There are a number of different ways in which the doctrine of divine simplicity can be formulated, but for dealing with criticisms like this one, the most useful is the logical approach. (It isn't a very useful approach for positive arguments for the doctrine, but it is very useful for showing how common criticisms of it fail.) A term within a proposition, like 'omnipotence', has a certain signification -- in this case, one involving power. It also has a supposition, i.e., a reference to something. So, for instance, when I say, "God is omnipotent," 'omnipotent' supposits for that in God which is supposed to be omnipotent. (In general, there are three kinds of supposition: material, simple, and personal. In material supposition a term supposits for itself, e.g., 'The word 'omnipotent' has four syllables'. In simple supposition, a term supposits for a universal or abstract quality or form, e.g., 'Omnipotence is a kind of power'. In personal supposition, a term supposits for that of which it is truly predicated, e.g., 'This omnipotent is God'.) Now the doctrine of simplicity put in these terms is just the following:
For any divine-attribute terms A and B, that for which A supposits is exactly the same as that for which B supposits.
Seen in that way, most criticisms of divine simplicity dissolve entirely, because they can be shown to confuse sense and reference. That's not to say it's a cure-all; there are objections it doesn't meet. But most of the criticisms that try to show that the doctrine involves some serious incoherence are themselves confusing signification with supposition. The deeper problem is that most critics of the doctrine (and some suppporters) don't recognize that the doctrine is, in fact, a step in negative theology. It does not make a commitment as to the real character of the divine substance; it merely denies that certain distinctions, and terms indicative of these distinctions, cannot properly be attributed to the divine nature, whatever the divine nature may be.
In any case, here is the comment I left at FQI, which I post here because it says something also about the importance of immutability:
I've never been impressed by Plantinga on simplicity; I think he's just confused on the subject. Part of the problem is perhaps the mistaken idea that the modern label 'property' introduces any precision into the discussion -- in fact, the reverse is true, because the modern notion is a catch-all, not a sharp notion. There is no generally conceded modern account of what properties are, and common usages typically lump together a number of things (accidents, properties, natures, sometimes even relations) that the medievals would regularly have kept distinct, and for good reason. The medieval discussion, which is usually in terms of signification and supposition (roughly, sense and reference) is much better.
When we put it in these terms, I'm not convinced that the problem arises. What is identical is not (say) perfect love and omnipotence; rather, what is one and the same is that to which we refer when we talk about God's love and His omnipotence. Now, nothing in this implies that the effects of the divine love and omnipotence are necessarily same across all possible worlds; only that, necessarily, in every possible world, that to which 'omnipotence' refers is one and the same as that to which 'divine love' refers. (That's why the truthmaker approach noted in a comment above is more promising than any property approach. The problem doesn't arise on a truthmaker approach, either, as far as I can see.)
It also seems to me that your argument isn't really against simplicity at all, but against immutability. But immutability isn't necessity: that God's will is immutable doesn't mean that it couldn't be otherwise than it is, but that there is nothing to which it is potential (i.e., there is nothing capable of making God actual in any way, because His will is actual prior to any candidate for making it actual). In other words, God's immutability doesn't contradict his freedom, but preserves it, by making it impossible for anything other than God to limit it. As Aquinas says somewhere, immutability is the strength of God; and Aquinas is right: it's the negative complement of divine strength, in the sense that it is the negation of what could limit divine strength or cause divine weakness. Eliminate the one, you eliminate the other.
