  Using a combination of analytical techniques and Quantum Monte Carlo
simulations we investigate the coupled spin ladder system LaCuO2.5. At a
critical ratio of the interladder to intraladder coupling (J'/J)_c \approx 0.11
we find a quantum phase transition between a Neel ordered and a disordered
state. At criticality the uniform susceptibility behaves as \chi(T)=aT^2 with a
universal prefactor. At intermediate temperatures the system crosses over to a
``decoupled ladders regime'' with pseudo-gap type behavior, similar to
uncoupled ladders. This can explain the gap-like experimental data for the
magnetic susceptibility of LaCuO2.5 despite the presence of the long range Neel
order.
