RAT5, I am not 100% sure but word has it that the ICAO + JAA regulations use the word "year" without defining what "year" means. This opened a useful gap into which our friends at Ryanair readily stepped. I am told that IACO always took it to be a "rolling year" but that when the matter is interpreted by an Aviation Authority it is up to them to decide what "year" means.
Word has it that Ryanair persuaded the IAA that one definition is as good as another (rumour, which is worth little, has it that the IAA did not quite realise what they were agreeing to). So no rules have been broken, apparently. However, I am not aware of any other Aviation Authority who have made a similar decision, but I am not an expert. It is also worth remembering that the 900 limit common in Europe is not a world wide limit. As I understand it several countries have a maximum annual limit of 1,000 hours (e.g. South Africa, the U.S.).
As a matter of interest does anyone know of any other exceptions to the "rolling year" and "900 hours" maxima?