When the total network hashing power get higher you get less BTC for your shares.
So if the BTC stays at the same price but the network speed gets 10 times as fast, and normally you need 10% of your BTC to pay the bills for the electricity, you now need all of your BTC to pay for electricity.
FPGA's are about 10 times as cheap in electricity consuming so they will rise from 1% of your BTC to 10% of your BTC.
Well the network isn't going to get 10 times as fast. However, blocks will give 25 btc in Dec 2012. At that point, at the same USD price, yes FPGAs will make a little more money then gpus. If the USD price goes up, GPUs are still better. At $6 it's about even (once blocks give 25 btc).
But that's 9 months out, and best case is FPGAs pull even or slighty ahead. The resale risk, BTCs USD value risk, if I had 10k to spend I'd still go with GPUs.