To clap or not to clap?

It's one of the most vexatious issues in classical music - when should you applaud and, even more importantly, when should you not applaud?  Modern concert-going etiquette is that a piece of music should be heard out in silence, with applause at the end.  The problem arises when the piece in question is divided into movements - as most symphonies and concertos are.  It is considered "bad form" to applaud between movements, and yet it was not always so.  Back in the 18th century (Mozart and all that) applause between movements and even during movements was not only accepted, but even to an extent expected.  It seem that it was towards the end of the 19th century that certain composers - notably Mahler and Wagner - made it clear that they wished their works to be assessed - and applauded if felt appropriate - as a whole.  The one place where applause between movements is still tolerated is at the Proms, which still tend to cling to the tradition of informality which was their original raison d'être.