The extreme limit of the generalized Chaplygin gas
  Unified Dark Matter models describe Dark Matter and Dark Energy as a single
entity which is, in the simplest case, embodied in a perfect barotropic fluid.
It is a well-established fact that small adiabatic perturbations of Unified
Dark Matter have an evolution characterised by oscillations and decay which
provide predictions on the Cosmic Background Radiation anisotropies which are
in poor agreement with observation. In this paper we investigate the
generalised Chaplygin gas and we find that the Integrated Sachs-Wolfe effect
excludes the model for $10^{-3} < \alpha < 350$. We discuss the implications on
the background evolution of the Universe if large values of $\alpha$ are
considered. In this case, the Universe expansion mimics a matter-dominated
phase abruptly followed by a de Sitter one at the transition redshift $z_tr$.
Thanks to an analysis of the type Ia supernovae Constitution set we are able to
place $z_tr = 0.22$.
