FPGA's or ASIC's for LTC will never be as close to the power of bitcoin ASIC's, or in the price. The differences will be enermous.
That's what I have read, too.
There was a thread on either this forum or the Litecoin forum that a person with FPGA experience had succesfully built and tested an FPGA for Scrypt using . The result was something like 3KH/s and from everything that I read, he wasn't very optimistic about anything much better than that being acheived. Factor in electricity and it *might* be beneficial, but it just depends on how much the initial cost is...
Simple economics. Fixed costs + Variable costs. A more energy efficient miner (FPGA) will be beneficial in the long-run only if it saves more electricity/hash (variable) than the initial purchase (fixed). Another thing that I think must be included when talking about Scrypt mining, is that if a GPU can produce 2KH/s/USD (HD7950 @ 600KH/s @ $300) then without factoring the benefit of electricity, an FPGA would need to produce more than that. Although an FPGA is reprogrammable, it likely will not retain the same value as a high-end GPU if for some reason Scrypt mining because completely unprofitable, either through waning interest or some ungodly ASIC development.
But, if someone could make an FPGA that produced >2KH/s/USD then I think it would be a hit. As it is, the Bitcoin USB Miners are a hit because of their sexiness and the fact that they are cheaper than a graphics card that will produce the same hashes. Even if reaching a breakeven ROI is a fantasy, it is a cool thing for a hobbyist to have. I think if that can be acheived with Scrypt mining through an FPGA or an ASIC then it would be met with much applause.
I highly doubt that you will ever see an ASIC for Scrypt that will hash at >50MH/s and be as compact as the 50GH/s+ ASICs for BTC.
I think this is the board that was used in the thread I read, but I didn't save all of the information because it didn't seem like a feasible option.
http://www.dinigroup.com/new/DNK7_F5PCIe.php