Can I offer one argument why this information might not be released at the moment.
Suppose he has (as has been suggested) committed a serious crime. If so I would hope that he would be charged with this crime, not just returned to prison. If, as has been claimed, he has committed a serious sexual assault, and he was put on trial under his new name, how long would it take for the media to realise that the 27 year old Joseph Bloggs charged with, say, rape was the same person as Jon Venables returned to prison for the identical offence. Thus it could be argued that he couldn't get a fair trial and he would have to be released, which is hardly what the public would want.