Nice detective work, ngzhang and xaxik.  I really did not expect with this type of community that the BFL would stay shrouded in mystery for too long.

the answer is, FPGAs include mass of possibility. if it can hold the Tj under the rated absolute maximum, i must say there is no over-driving at all.

Gimme some of what your smoking 

Spartan 6's do not go for $240 even at retail more like $175 retail

have you ever heard of getting things cheaper in bulk? 

I imagine you could get them $100 ea or cheaper in QTY

How many chips would you have to buy? IDK I figure if your buying thousands you can get a pretty sweet deal though

That is a very strange warranty period for something that is considered a long term (> 1year) investment.

Likely: Nil.

And worse than the longer warranty that some other products offer.

True, true. But at what point does the additional cost cover the extended warranty?

But how can they get so high mh/s compared to a $200 Spartan-6 chip?

True - but also at what point does the lost BTC from waiting for your device to arrive cover the lower price.
i.e. if you spend 10 weeks waiting for a paid for 830MH/s device - that's currently 0.55338097BTC x 70 = ~38.7BTC or currently ~$193
So if you can get a similar device that mines at 830MH/s today now, then you could effectively cut $193 off the price before comparing.
You can of course make that time frame longer or shorter depending on how long a BFL really takes to get (longer) or shorter by subtracting the time another device takes to get

Yeah, anyone who can write a firmware for it to work