Straight after the GeForce GTX 590 was released the first reports came in from overclockers that pushed a little bit too much voltage into the GPU, making the card in some cases die. It seems that the power design of the card is so weak that NVIDIA quickly locked down Voltage adjustment in this card series.
And for an enthusiast class product, that makes it a little well a little bit of a lackluster ?
The latest tail from the web however is that in June a revised model will be released, this updated version of the hardware will get a much more sturdy VRM circuitry, mosFETs, capacitors, etc.
Once the new revision is out we'll be certain to test it and see of it can clock much better and thus compete a little more fiercely with the Radeon HD 6990, that does allow its ass to be kicked in terms of overclocking.
Update: there is no revision of the GTX 590 coming. This is simply a change in component source supplier.
Official quote from NVIDIA;