Where's the line?

Alright, maybe it's been a while since I posted a new blog. But let's put that behind us now and move on, yes?

A while ago, I got a voicemail from a listener who was displeased with a particular selection on midday classical. The piece was from Ludovico Einaudi's album Divenire and it featured piano, loops and cello. The listener (who, sadly, did not leave a phone number) said that just because a piece has cello and piano doesn't mean it is classical music. I wish I could have called that listener back because I wanted to ask what makes classical music. I find that question to be endlessly fascinating and probably impossible to answer fully--at least for me.

What do you think? Do the limits of classical music lie in instrumentation? Form? Structure?

Plenty of music we now call "classical" was "popular music" when it was written--or at least based on popular music. So when does it become classical music? Is this a temporal issue? Does a piece have to be a certain age before it becomes classical music if it evokes or includes a popular idiom?