The much-hyped “Concert Companion,” a PDA-sized device which allows concertgoers to follow a written commentary to symphonic works live in the concert hall, is becoming the gotta-have-it item for major American orchestras. With successful trials in New York and Pittsburgh now complete, other orchestras are lining up for the chance to try out the device on their audiences. In fact, it may be hard for the supplier to keep up with demand, not so much for the hardware, but for the specialized commentary (written, up to this point, by ArtsJournal blogger Greg Sandow) which must be composed for each featured work.