  We exploit the quantum coherence between pair-produced D0 and D0bar in
psi(3770) decays to study charm mixing and to make a first measurement of the
relative strong phase \delta between D0 -> K+pi- and D0bar -> K+pi-. Using 281
pb^-1 of e+e- collision data collected with the CLEO-c detector at E_cm=3.77
GeV, as well as branching fraction input from other experiments, we make a
preliminary determination of \cos\delta = 1.03 +- 0.19 +- 0.08, where the
uncertainties are statistical and systematic, respectively. By further
including other external mixing parameter measurements, we obtain an alternate
measurement of \cos\delta = 0.93 +- 0.32 +- 0.04, where the systematic
uncertainty from assuming x\sin\delta=0 has not been included.
