If you had the 600gh/s card you could make 1BTC in 5.5 days. That's a ROI in only 11 days.  If you really think BFL is going to hand you such hardware for 1.9BTC when difficulty is still hovering in this range then you are deeply mistaken.

What happens is that you will fund the hardware, BFL gets it built with YOUR money,  mine the crap out of it and ship it to you when 600GH/s makes one BTC in 550 days. Very simple.

Also if you build a GPU miner you will have good resale value let's say 50% of what you put in.  With dedicated hardware your resale value is pretty much zero when the thing can't make BTC faster than one in 1800 years.  See what happened to those $60 USB miners? Worthless now.