But I do want to comment on your post, I am not sure what you mean by custom rig? every rig is a custom rig if you think about it, and I agree with you the days where people are going to invest in new gpu's for mining is gone unless of course we see a BTC value increase like we did when it went up to $30, thats what started the first one.
There are two methods to mine. One is "casual mining". Hey I like video games. I got a high end rig. I can mine when I am not using it and earn $30 to $60 per month. All the hardware is dual use.
The other method is custom rigs. Rigs built to do mining and nothing else. USB sick, open frames (or cheap cases), sempron CPU, little ram, high end CPU, and as many GPU as you can cram in.
"casual mining" has the advatange that the hardware is "free" (the game would own the hardware even without bitcoin). Custom rigs hve the advantage of scalability.
Most miners started as "casual miners". Some never moved past that. However some miners (like myself) have and while less common the throughput of these hashing farms mean they produce a lot of hashing power in aggregate.
So something (anything) that affects dual-use systems (casual miners) will have absolutely no effect on a mining farm.
Also FPGA mining is a neat idea and is a much more energy efficient choice but again anyone that makes a major FPGA commitment is going to have to invest major cash into something that is not going to pay off well (unless the value goes up as my statement above)
The issue w/ FPGA isn't price but stability. If I knew that adjusted for difficulty BTC would be >$2 in 4 years (i.e. difficulty / price <= 500,000) I would buy 20K worth of FPGA today. With my electrical cost it would be an ROI of 35% annually if I averaged $2 per BTC (@1 mil difficulty). Kinda hard to find 35% ROI that is scalable. The issue isn't price but volatility. If (adjusted for difficulty) over the next 4 years price of BTC averages $0.10 well I would lose a tons of cash. That risk is hurting adoption of FPGA. FGPA aren't going to make anyone 10,000% profit a year but thinking you can make 10,000% profit annually by leaving your computer running is foolish. FPGA have the ability to produce REASONABLE profit if prices just stay level however that remains to be seen. Unlike a GPU FPGA have little or no resale value so the capital risk is much higher.
FPGA miners don't need higher prices they need assurance that prices won't go significantly lower.