If you visit the John Addis Gallery of Islamic Art at The British Museum, what you are actually seeing are objects made in lands where, at the time that the artifacts were made, the ruling body embraced Islam. Notwithstanding the great patronage of many of these rulers, which has produced a vast array of fine architecture and art, a good percentage of the pieces were made by, or for, non-Muslims: and several of the objects, for example wine cups, would not be associated with a Muslim way of life today.
The first collections of 'Islamic Art' as we know it today started to be assembled in the 19th century, with the Duc de Blacas, a French nobleman, being one of the first collectors to really acknowledge the worth of these pieces alongside classical artefacts. Interest grew, until it reached a pitch at the time of the Arts and Craft movement, whose practitioners admired and sometimes replicated the techniques, forms and themes of such things as lustre pottery and enamelled glass. Amongst collectors and admirers there was a certain amount of ignorance about the actual origins of some of the techniques and the objects themselves, and the scholar Oliver Watson would also argue for a degree of intellectual snobbery, with anything particularly fine being labelled (often erroneously) 'Persian', since this was somehow perceived to be more sophisticated than 'Arab'. This particular trait is still clouding the scholarship of lustrewares today.
So, back to Islamic art. Is it a valid term at all? For certain objects I think it is: take for example the beautiful Iznik mosque lamp made for the refurbishment of the Dome of the Rock by Suleyman the Magnificent. Mosque lamps have a distinctive form, being designed to hang in numbers from the ceiling of the mosque, and can be fashioned from glass, pierced metal or ceramic with a lit wick in oil casting the light. It is clear from the use of pottery, which clearly does not let the light shine through it, that the lamps had a symbolic as well as a functional purpose. This is underlined by the fact that many lamps are inscribed with the Surah, or Chapter, of Light from the Quran, in which Allah is likened to a light and is the only metaphor for God in the Quran. This particular example is inscribed with the Shahada, the Muslim profession of faith and the first of the Five Pillars of Islam. The combination of the form and the use of Arabic calligraphy, which acquired almost the sanctity of a sacred script due to the fact that it was via Arabic by which the prophet Mohammed received the revelations of the Quran, give the object a meaning far beyond its function and associate it strongly with the Islamic religion. This, I believe, makes this particular object truly Islamic art, but such strong associations cannot really be applied to the majority of objects found in an 'Islamic Gallery' and the lumping together of all objects found in such a wide range of countries (Spain to India) and across so many centuries (7th AD to the present) seems to me to be a construct of 19th century Europe which needs revision today.