  We re-examine the Peierls insulator to Mott insulator transition scenario in
the one-dimensional Holstein-Hubbard model where, at half-filling,
electron-phonon and electron-electron interactions compete for establishing
charge- and spin-density-wave states, respectively. By means of large-scale
density-matrix renormalization group calculations we determine the spin,
single-particle and two-particle excitation gaps and prove--in the course of a
careful finite-size scaling analysis--recent claims for an intervening metallic
phase in the weak-coupling regime. We show that for large phonon frequencies
the metallic region is even more extended than previously expected, and
subdivided into ordinary Luttinger liquid and bipolaronic liquid phases.
