  The hydrodynamic expansion of quark-gluon plasmas with spherical and
longitudinally boost-invariant geometries is studied as a function of the
initial energy density. The sensitivity of the collective flow pattern to
uncertainties in the nuclear matter equation of state is explored. We
concentrate on the effect of a possible finite width, $\Delta T \sim 0.1 T_c$,
of the transition region between quark-gluon plasma and hadronic phase.
Although slow deflagration solutions that act to stall the expansion do not
exist for $\Delta T > 0.08 T_c$, we find, nevertheless, that the equation of
state remains sufficiently soft in the transition region to delay the
propagation of ordinary rarefaction waves for a considerable time. We compute
the dependence of the pion-interferometry correlation function on $\Delta T$,
since this is the most promising observable for time-delayed expansion. The
signature of time delay, proposed by Pratt and Bertsch, is an enhancement of
the ratio of the inverse width of the pion correlation function in
out-direction to that in side-direction. One of our main results is that this
generic signature of quark-gluon plasma formation is rather robust to the
uncertainties in the width of the transition region. Furthermore, for
longitudinal boost-invariant geometries, the signal is likely to be maximized
around RHIC energies, $\sqrt{s} \sim 200$ AGeV.
