A gold 1000 btc casascius coin was sold for 1 million USD.
That was buying some bitcoin, not buying with it, and it was paid for with gold and USD.
It would be interesting to see if someone like Bitpay has an upper limit on purchases you can make. There must be a point where they can't guarantee that they'll be able to put it through without losing out.
Ideally it wouldn't be converted back. That being a common occurrence will hopefully happen reasonably soon.