  We investigate the conditions under which a gap vanishes in the spectrum of
dimerized coupled spin-1/2 chains by means of Abelian bosonization and Lanczos
diagonalization techniques. Although both interchain ($J'$) and dimerization
($\delta$) couplings favor a gapful phase, it is shown that a suitable choice
of these interactions yields massless spin excitations. We also discuss the
influence of different arrays of relative dimerization on the appearance of
non-trivial magnetization plateaus.
