According to the official webpage of Buckingham Palace; HM the Queen, HRH the Duke of Edinburgh, HRH the Princess Royal, HRH the Earl of Wessex, TRH the Duke and Duchess of Gloucester all have engagements the days Queen Sonja celebrates her 70th birthday. This can be a sign of that HRH the Prince of Wales and HRH the Duchess of Cornwall is going to Norway - since they have no engagements mentioned. But still, we just have to wait and see.