prospects for FPGA is indeed not so bright. If difficulty will grow near 18% in two weeks (it been growing around 15 % in 2 weeks recently) then by November 2013 diff will be 96203242 that means 0.08 btc per month from 500mgh FPGA unit.
Even for ASIC it will cut profit quite significantly, after last diff rise from 11 to 12 mills my bitcoin mining capacity reduced from 10 to 8 btc per day
hope that btc rate will grow in same pace and bitcoin will not cost near 700usd by November. It may cost that much in fact, as we've seen with previous hash rate growth, btc rate been on rise too. In this case hoarding btc (or investing into ASICMINER shares) can be more profitable rather spending it on buying into FPGA mining.