  We present a spin-1/2 bilayer model for the quantum order-disorder transition
which (i) can be solved by mean-field theory for bulk quantities, (ii) becomes
critical at the transition, and (iii) allows to include intralayer frustration.
We present numerical data (for systems with up to 240 sites) and analytical
results for the critical coupling strength, ground-state energy, order
parameter and for the gap. We show that the critical coupling decreases
linearly with frustration.
