My Excel tells me that for Litecoins I need less Mh/s to reach a target of x USD per day revenue.
You will not get the same Mh/s mining LTC that you do for BTC on any device. Thus, mining Bitcoins on any current FPGA is more profitable, and is the sole currency people are buying FPGAs for.
ie, you may get 360 Mh/s mining BTC on a 6950, but the same card will only yield 0.4 Mh/s or 400 Kh/s if mining Litecoins.
I'm not even sure if the current FPGAs can run the script for LTC mining.