As a final thought: I don't see why Darwin is necessary here. That is, I don't see why bringing Darwin(ism) into the picture allows one to get out of idealism. Surely one could be both a Darwinian and a transcendental idealist -- one would simply say that Darwinism is merely empirically real, and the truth (or falsity) of transcendental idealism is untouched.
Carl’s comments above seem to me to be based on a misunderstanding of Habermas’ position (which is most probably due to my own poor formulation of Habermas’ position). Anyway Carl’s comments provide me an opportunity to try to understand Habermas’ position in a better way.
To start with, Habermas is not aiming to overcome idealism. For Habermas, idealism has been decisively refuted. It has become untenable. It has lost its persuasive force etc. Similarly, Habermas is not aiming to prove Darwinism or naturalism in general. He takes the belief in naturalism to be the cumulative effects of learning process of the last two hundred years (not only in philosophy but in sciences but also at the level of culture in general).
So what’s Habermas aiming at? I think, Habermas' aim is to reconcile ‘transcendentalism” and Darwinism. Transcendentalism is a position that involves many things but particularly a commitment to the notion of embedded rationality and the view that every understanding must spring from within our own lifeworld even if it will eventually have resources also to go beyond it. Habermas treats this sort of ‘transcendentalism’ as an irrefutable fact which he thinks we have (or should have) learnt form Kant, Hegel, Heidegger and others. Such a transcendentalism has its own problems. One of them is that it leads to (or may lead to) an anthropocentric version of rationality, which Habermas wants to avoid. Such a transcendentalism also conflicts with Habermhas’ equally strong commitment to naturalism which requires giving ontological priority to “nature” (considered broadly). Hence the whole business of reconciling Kant and Darwin. Kant here is a stand-in for the sort of transcendentalism explained above while Darwin is a stand-in for ‘naturalism.’
Now I don’t claim that such a project or vision is without its problems. However, as far as I understand him, this is the view espoused by Habermas and it interests me because it has philosophical repercussions which are quite general.