IMDB Board Prediction
The IMDB plan might be to delete all the data and start again - rather than deleting the forums forever. If they bring it back, you'll have to use your credit card to join for identity purposes.
Recently, anonymous people on Twitter have been brought before the courts for cyber harassment, making racist remarks, threats etc. It maybe IMDB are thinking if somebody says this actor is sleeping that actress, or somebody says they'd like to shoot a celebrity dead, the name and address of that person might have to be given to the authorities for prosecution. IMDB would also want make sure they're are not accountable for what people say on their forums in the terms of agreement...