The annihilation or decay of Dark Matter (DM) particles could affect thethermal history of the universe and leave an observable signature in CosmicMicrowave Background (CMB) anisotropies. We update constraints on theannihilation rate of DM particles in the smooth cosmological background, using WMAP7and recent small-scale CMB data. With a systematic analysis based on the Press-Schechter formalism, we also show that DMannihilation in halos at small redshift may explain entirely the reionization patterns observed in the CMB, under reasonable assumptions concerning the concentration andformation redshift of halos. We find that a mixed reionization model based onDM annihilation in halos as well as star formation at a redshift z ≃ 6.5could simultaneously account for CMB observations and satisfy constraints inferredfrom the Gunn-Peterson effect. However, these models tend to reheat the inter-galactic medium (IGM) well above observational bounds: by including a realistic prior on the IGM temperature at low redshiftand allowing most of the reionization to be due to star formation, we find stronger cosmological bounds on the annihilation cross-section than with the CMB alone.