ORIGINAL: siclick33
The problem with a poll at checkout is that, as I mentioned earlier, it depends on what bit is backordered.
It's only a problem if you don't offer the right choices.
As an extreme example,
I order an engine and some cyano but the cyano is not in stock. I would want the engine straight away but would cancel the cyano order.
I order the same items but the engine is not in stock. Now my choice would be different; I would want to cancel the whole order.
So, just have the choice at checkout incldue the option "If anything is back-ordered, contact me before proceeding with the order".