I don't think he's published it yet; he did it in an internal FHI meeting. It's basically an extension of the fact that an uncomputable algorithm looking only at programmable models can't find itself in them. Computable versions of AIXI (AIXItl for example) have a similar problem: they cannot model themselves in a decent way, as they would have to be exponentially larger than themselves to do so. Shortcuts need to be added to the algorithm to deal with this.