I don't think it's fair to say that it's entirely arbitrary. The 1000$ mark holds significant psychological impact. When the price rises, it's going to be reported on, and it makes sense for it to be reported on when it hits a psychologically important value like 1000$ rather than 947.13$
I don't understand this. "1" BTC is an arbitrary amount as well. Perhaps in the long run, mBTC or uBTC will be the perspective we hold of Bitcoin. From some of what I've read, that was sort of the point of the high divisibility.