If I understand what Kea said here recently, Laurent Freidel and Aristide Baratin have a paper in the works about BF theory of 4D spacetime with matter put in as topological defects in the gravitational field (like places where space is somehow tangled) To do this one needs the theory of 2-groups----at least one needs the Poincaré two-group. So the Baratin-Freidel paper will have two-groups. Baez had a thread here at PF a year or so ago telling us about this. There are some talks about it at his website. So a Baratin-Freidel paper was expected about this, but, to add to the excitement, this paper from Hendryk Pfeiffer comes out url]http://arxiv.org/abs/0708.3051[/url] Topological Higher Gauge Theory - from BF to BFCG theory Florian Girelli, Hendryk Pfeiffer, E. M. Popescu 15 pages (Submitted on 22 Aug 2007) "We study generalizations of 3- and 4-dimensional BF-theory in the context of higher gauge theory. First, we construct topological higher gauge theories as discrete state sum models and explain how they are related to the state sums of Yetter, Mackaay, and Porter. Under certain conditions, we can present their corresponding continuum counterparts in terms of classical Lagrangians. We then explain that two of these models are already familiar from the literature: the SigmaPhiEA-model of 3-dimensional gravity coupled to topological matter, and also a 4-dimensional model of BF-theory coupled to topological matter."