While I am not certain how reliable the source is, it seems highly likely that Mr Cameron will be wearing a lounge suit for Prince William's wedding next Friday, as will Mr Clegg. If true, this is very disappointing news.
It is true that the wedding invitation permits 'Uniform, Morning Dress or Lounge Suits'. However, this was also the case at Prince Charles's wedding in 1981, and yet as far as I can tell every male politician in attendance wore morning dress. Clearly, the then Prime Minister did not, but her husband did and so did Mr Whitelaw, the deputy Prime Minister (immediately to the right, and second to the right of Thatcher, respectively.)
So what has changed, and why does a man who was educated at Eton and Oxford, and is notorious for spending much of his time at the latter swanning around in the custom evening tails of the Bullingdon club, think it is appropriate to wear a lounge suit to the wedding of the future King of England? It is almost certainly the most formal wedding he will ever attend, and there is absolutely no doubt that morning dress is the most appropriate attire for a civilian. Not to wear it is to make a deliberate statement, whether Cameron intends it or not. It will make his appearance at the wedding look more like just another political engagement, for which he has not even bothered to change out of his safe politician's uniform of a plain dark suit, a plain light shirt, and a plain coloured tie.
Why is he doing it? Because he is terrified of public opinion and terrified especially of the prospect of a photo of him in a top hat and tails appearing alongside every store about his privileged background, for the rest of his time in office. To that, I would say that he needs to have the courage of his convictions (or, at least, the courage of his upbringing) and confidently dress for the occasion.