May never ROI. Since difficulty can fall (or stop rising), there are infinite scenarios under which a BFL Single could generate ROI.
However, the more BTC rises in USD, the better off the early pre-order people would have been just buying BTC from Gox or BTC-E instead of BFL.
In almost every case, buying bitcoins would have been smarter, easier and more lucrative than buying ASIC mining hardware.
That is a separate subject from whether or not my BFL hardware returns the initial investment (it will).