D-Sphalerons and the Topology of String Configuration Space
  We show that unstable D-branes play the role of ``D-sphalerons'' in string theory. Their existence implies that the configuration space of Type II string theory has a complicated homotopy structure, similar to that of an infinite Grassmannian. In particular, the configuration space of Type IIA (IIB) string theory on $\R^{10}$ has non-trivial homotopy groups $\pi_k$ for all $k$ even (odd). 
Jeffrey A. Harvey, Petr Horava and Per Kraus