In The Guardian, fuller version on his own site.

[update: some interesting comments on the New Apps blog. This obviously links to previous discussions of the topic. See, for example, my comments here. The final paragraph of that is, I think, crucial. If we want to change the system it has to be more than simply breaking existing rules (uploading our own material ‘illegally’ or trying to change things bit by bit). It would need, as is said in the New APPS post, a change in behaviour about where we publish, but also, perhaps more crucially, a change in assessment, so that appointment, tenure, promotion and research assessment panels saw a piece that was in an open-access, and/or non-major publisher outlet as potentially equal to one in more standard outlets. But that would require, at least, a willingness on their part to read the material.]