Observation of B0 -> K*0 K*0bar and search for B0 -> K*0 K*0

We report the observation of the b -> d penguin-dominated decay B0 -> K0 K0bar with a sample of 383.2 +/- 4.2 million BBbar pairs collected with the BaBar detector at the PEP-II asymmetric-energy e^+e^- collider at the Stanford Linear Accelerator Center. The measured branching fraction is Br(B0 -> K0 K0bar) = [1.28^{+0.35}{-0.30} +/- 0.11] x 10^{-6} and the fraction of longitudinal polarization f_L (B0 -> K0 K0bar) = 0.80^{+0.10}{-0.12} +/- 0.06. The first error quoted is statistical and the second systematic. We also obtain an upper limit at the 90% confidence level on the branching fraction for Br(B0 ->K0 K0) < 0.41 x 10^{-6}.