  The derivation of a Moving Boundary Approximation or of the response of a
coherent structure like a front, vortex or pulse to external forces and noise,
is generally valid under two conditions: the existence of a separation of time
scales of the dynamics on the inner and outer scale and the existence and
convergence of solvability type integrals. We point out that these conditions
are not satisfied for pulled fronts propagating into an unstable state: their
relaxation on the inner scale is power law like and in conjunction with this,
solvability integrals diverge. The physical origin of this is traced to the
fact that the important dynamics of pulled fronts occurs in the leading edge of
the front rather than in the nonlinear internal front region itself. As recent
work on the relaxation and stochastic behavior of pulled fronts suggests, when
such fronts are coupled to other fields or to noise, the dynamical behavior is
often qualitatively different from the standard case in which fronts between
two (meta)stable states or pushed fronts propagating into an unstable state are
considered.
