Assuming they do start to deliver, the first recipients will be the only ones that have any chance to pay off their device. Soon after delivery they will not be able to mine more than the best GPU can today.
What are you basing that on?
Please remember that as diff goes up, so will the value of BTC, if it doesn't then no one will mine.
Who is going to run a 50gh unit when the diff is 300m if BTC is $140? No one that is who!
But if BTC is $1000 and you earn 1/10th of a BTC a day, that doesn't suck.
I'm basing it on the fact that difficulty is going to skyrocket and the cost per coin needs to jump astronomically to keep up and make it profitable. You better hope trade volume increases a lot and btc does jump to $1,000 quickly. I don't see the logic in anyone not mining with a device that's energy efficient even as price falls. They waited so long and have such a large investment in hardware it would be stupid to ever shut it off. It's not like you can repurpose it or play video games with it.