I think it's borderline ridiculous to think a scam would first start with the heavy engineering required to develop FPGA's just as a trick to hook us for a larger ASIC run. If you think they started out with the intention to scam, well then I think they've realized they've stumbled onto a "more profitable if legitimate" business. Developing FPGA's is not trivial, nor is the amount of engineering that has gone into BFL's already developed products. Not only that, BFL also has competitors- some of whom also are offering pre-orders for ASIC's by the end of the year. A competitor could have at any point scooped BFL by announcing their ASIC's first or developing them earlier
It doesn't require that much heavy engineering; I did a fair bit of work on bitstreams for one or two of the competing boards despite having no prior FPGA development experience whatsoever, and I believe someone else has been building a mining farm out of home-built FPGA boards soldered on a hotplate. The FPGAs that BFL are using are a bit trickier on the board-design side but probably easier on the bitstream-development side, so really it all balances out in the end.