  The intrinsic linewidth $\Gamma_{TA}$ of the transverse acoustic (TA) phonon
observed in the relaxor ferroelectric compound
Pb(Mg$_{1/3}$Nb$_{2/3})_{0.8}$Ti$_{0.2}$O$_3$ (PMN-20%PT) begins to broaden
with decreasing temperature around 650 K, nearly 300 K above the ferroelectric
transition temperature $T_c$ ($\sim 360$ K). We speculate that this anomalous
behavior is directly related to the condensation of polarized, nanometer-sized,
regions at the Burns temperature $T_d$. We also observe the ``waterfall''
anomaly previously seen in pure PMN, in which the transverse optic (TO) branch
appears to drop precipitously into the TA branch at a finite momentum transfer
$q_{wf} \sim 0.15$ \AA$^{-1}$. The waterfall feature is seen even at
temperatures above $T_d$. This latter result suggests that the PNR exist as
dynamic entities above $T_d$.
