  The scaling of the conductivity at the superfluid-insulator quantum phase
transition in two dimensions is studied by numerical simulations of the
Bose-Hubbard model. In contrast to previous studies, we focus on properties of
this model in the experimentally relevant thermodynamic limit at finite
temperature T. We find clear evidence for deviations from w_k-scaling of the
conductivity towards w_k/T-scaling at low Matsubara frequencies w_k. By careful
analytic continuation using Pade approximants we show that this behavior
carries over to the real frequency axis where the conductivity scales with w/T
at small frequencies and low temperatures. We estimate the universal dc
conductivity to be 0.45(5)Q^2/h, distinct from previous estimates in the T=0,
w/T >> 1 limit.
