Edited: Original calculations overstated BTC/day for both FPGA and GPU.  Revised calculations show that FPGA makes sense at a much lower power cost.

It seems that the viability of this product rests completely on electricity pricing.  If you compare FPGA to GPU with the same amount of money invested it appears (using round numbers) that somewhere in the range of 12 8 cents per kWh is where either solution is roughly equal in profit.  If your power costs more than that, you're better off with an FPGA.  If you can get cheaper power, go for GPUs.

As an example, let's say you invest $2000 into mining.  With that, let's assume you could either buy 5x ZTEX FPGA modules or two GPU mining rigs with 8x GPUs.

FPGA
Investment: $2000
FPGA's: 5
Total Mh/s: 950
Monthly power cost at $0.08 for 45W: ~$3
BTC Payout: 0.6508 BTC/Day or ~20 btc/month
USD Payout at $2.70: $54
Monthly net revenue from $2000 investment (BTC minus power): $51

GPU
Investment: $2000
4-GPU rigs: 2 (each having 4 x Radeon 5850 GPUs producing 333 Mh/s each)
Total Mh/s: 2664
Monthly power cost at $0.08 1700W: ~$99
BTC Payout: 1.825 BTC/Day or ~55.5 btc/month
USD Payout at $2.70: $150
Monthly net revenue from $2000 investment: $51

Of course this comes with the standard caveats that BTC prices and difficulty will change and that you might be able to overclock GPUs and get even more Mh/s, etc. etc..  Also, if AMD can deliver on huge cuts in power consumption with the 7xxx series GPUs, the power indifference line will increase.

But, if you're considering investing in mining equipment and are looking at this FPGA or GPUs, your electricity rates may have already made the decision for you.