Do Not Ban
First of all, I just want to say that the amount of time given to determine whether or not Cresselia is ban worthy is inadequate. All this theorymoning about how Cresselia will be too much for the metagame is pointless. Lilligant and Honchkrow were first thought to be too powerful, but eventually everyone adapted without having to overprepare. Honko's post made in the Cresselia discussion thread led me to believe that this might be possible for Cresselia if given the time.
As for the Pokemon itself, it has caused an immediate spike in the usage of dark type pokemons. Almost every team I faced either had Spiritomb or Drapion, sometimes even both. Personally I think this is an overreaction to Cresselia returning to RU as everyone believed that it was too powerful the first time therefore it is too powerful this time around. If given enough time, better strategies might arise instead of stacking Dark type pokemons.
My experiences with Cresselia has also influenced me to not kick Cresselia out of the tier yet. It's not like Cresselia is sweeping through teams with ease, although this may be due to the overpreparation that I mentioned earlier. Each of its sets has its disadvantages so Cress cannot be labeled as the undisputed best pokemon for the job. Its Dual Screen set is slower than Uxie and doesn't offer SR support. Mespirit can match Cresselia's Lunar Dance with Healing Wish and offers SR support. Its offensive movepool is pretty bad and a CM set can be easily stopped with Toxic or through phazing since it lacks immediate power. The defensive set is probably where Cresselia shines the most since it can check a lot of things and doesn't get worn down by hazards. With Snover out of the way its easy to replenish its health too. However I don't think enough time has been given to see if this set really has the impact that all the hype leads us to believe it will have. Definitely does not deserve a quick ban.