But there has been cases that NON overclocked GTX590's died also, so yes there is a flaw in these cards.
I think both the 6990 and the 590 should be retested in about 3-4months time with better drivers etc, this then will tell us who is king of the hill??
These days, it would be better they don't bother with dual GPU pcb at all.
All of the benefits of dual GPU :
Quiter
Cooler
Cheaper(versus two separate cards)
Faster (if albeit only through bandwidth because the cores are simultaneously talking to the motherboard directly in unison)
No (or less) microstutter
Potentially higher scoring ceiling for synthetic benchmarks(TRI SLI, Quadfire etc.)
are not entirely true anymore - certainly not altogether at once.
The only remaining definite positive is that it only takes up one PCI E lane, but you'd be hard pressed to find a motherboard anymore(especially one suitable for high end computing) that comes with a single PCI E lane forcing you to buy a dual GPU solution as opposed to Crossfire/SLI.
Neither the 6990 or the 590 are impressive in performance, efficiency or cost and I would hope that any computer consumer looking to purchase enthusiast level products, would have half a brain and come to the conclusion that unless they are solely focused on synthetic records, then (as a current example) a 6950 Crossfire or 570 SLI setup is more effective at being reliable, faster (circumstancial and relative, i.e. if you're getting strong minimum frames 50-60 or higher), and generally more efficient overall in achieving it's goal.