Readers of gr-qc could have lost the preprint from Isham at quant-ph this January, Quantising on a category http://arXiv.org/abs/quant-ph/0401175 I guess it is related to his series A New Approach to Quantising Space-Time ( gr-qc/0303060 gr-qc/0304077 gr-qc/0306064 ) which is so abstruse that nobody has quoted it. Mutters about using Topos theory to describe quantization functors has being going around for a time, as well as other collateral links: Doplicher took some attemps to use category theory in a practical way, and even Moerdijk (coauthor of one textbook on sheaf theory) did some feints to Connes noncommutativity, in the context of foliations, of course. On the mathematical side, I have heard nothing about using topoi for foundational purposes since the homonimous australian book (in the seventies? fron Goldblatt?).