http://event.asus.com/server/2011/esc4000/

I'm not sure about heat dissipation, but it does have 8 PCIe x16 slots and 1400W PSU with optional redundant extra PSU...

I see only four slots for double slot videocards. And yes, it is meant for GPU calculations in first place, thats why it is preconfigured with Tesla cards. I can imagine how these machines can be stacked for serious computing power and maximum of 50 bitcoins every 10 minutes.Bitcoin price is not tied to difficulty. Only available supply and demand for bitcoins drives the price.

Yes, but mining payoff depends on price and many people (still!) assume, that if difficulty grows, price will go up too.

With the next increase in difficulty, if the price stays stable at ~6-8 USD/BTC, mining starts to get quite close to being unprofitable - even with low electricity costs.


Back on topic, it might however really be an interesting solution to have a 4U case (or any other "Desktop"-style case) with 8 GPUs arranged like in this case in the rear connected via PCIe extenders/multipliers.

Well, at a difficulty of 400000, 100MH/s give you ~25 bitcents per day.
Times 6 to 8 = 1.5 to 2 USD/day
You need to know your Watt per MH/s ratio yourself, but a ratio of 1 (including other peripherals/CPU/RAM...) is already quite ok.

This would mean that you use up 4*2.4 kWh to mine 1 BTC --> 9.6 kWh need to be worth less than 6-8 USD.

You can also do these calculations backwards to see at which difficulty you pay more just for electricity than what you get in BTC.