Artcoustic offers custom made in-wall speaker systems which can incorporate specially manufactured acoustically-transparent grills. These allow you to hide your speakers behind a work of art without affecting sound quality.
Artcoustic offers a variety of beautiful designs or you can even submit your own design for incorporation into your speakers. This is one of those great examples of form and function living in harmony. Prices for their Diablo Monitor speakers start around $2500 USD per pair with customized artwork.