As always the Washington Post"s Anne Midgette puts the picture into perspective:
As a genre, classical crossover, as defined by the Billboard chart, is pretty treacly stuff: the tenor Andrea Bocelli, chanting monks, the cellist Yo-Yo Ma airing his lighter side.
Yet more and more artists are exploring music outside their traditional boundaries, in ways that are more and more interesting to a discriminating audience. Our friend would never have put a Three Tenors CD on at a dinner party. But the Balanescu Quartet's album "Possessed," which came out in 1992, offered a lot more meat for the ear. In fact, the association with "crossover" -- the juxtaposition of different kinds and styles of music -- may have been a liability; once we knew what it was, we moved on to another topic more readily than we might have had we been told we were listening to, say, music by an obscure Romanian composer.
Broadly speaking, there are two kinds of crossover music: classical artists exploring other musical genres, and non-classical artists taking the plunge into classical. Each kind runs the gamut from schlocky (remember the rock albums by the German heldentenor Peter Hofmann in the 1980s?) to serious (indie-pop star Sufjan Stevens's orchestral/instrumental work "BQE").
For classical musicians, crossover projects generally imply letting one's hair down: Daniel Barenboim's 1996 "Mi Buenos Aires Querido" is a really fun recording. There's also the appealing prospect of reaching new, and supposedly larger, audiences (a la the Three Tenors). The pianist Christopher O'Riley has become identified with his solo arrangements of songs by Radiohead; he didn't start playing Radiohead to make money, but he's certainly had a bigger success with those songs than in more conventional classical repertory. (His recitals now often include both.)