Ok, before people tear each others heads off about OCing being a right vs. privilage; here is the low-down:
A company designs a chip to be ran at say 100Mhz. Now, due to production faults most chips can run at 100MHz, but some, say 20%, fail. So, the chip maker specs the chip at 85MHz, at which point 99.9% of chips work perfectly fine. The chips get sold to general public and overclocking madness starts. Some users ger their 80MHz chip and are able to run it at 100MHz, while other, less lucky folks cant run their chips over 90MHz. Now, extend that analogy to cards. GTX590 was designed to run at 600MHz (just a guess) so, the production spec for gtx 590 was going to be 525MHz. Then HD6990 arives and Nvidia realizes that at 525MHz the 590 can not compete. So, they factory overclock them to 600MHz and ship out to vendors. As the result, the end user has a card which has no safety cusion, or overclocing ability beyond the stock clock. Some cards blow up.
And Nvidia goes back to work to design new gtx590, for the sake of notation, lets call it gtx590NED (gtx 590 Non-Explosive-Design). The new card GTX590NED has better components and is designed to run at say 700MHz, so the factory clocked models at 600MHz have 100MHz OC possibility, for the lucky few that get good cards, which were not in the 20% of units failing at 700MHz.
So, minus the bad original PR everything seems good and solid. Well not as solid as I wish it was. Reason being is that the new card, GTX590NED will support overcocking better, however I bet its thermal solution, which was originally designed for 600MHz mark was not reworked, just the base plate changed to account for new parts. So, while it should still work well at the 600MHz that the cards are shipped at it probably will not work well at 700MHz, which the new cards should be overclockable to, because the thermal capasity equivalent to 600MHz clock was already the target design for thermal solution, so what will its performance be at 700MHz that someone tries to OC gtx590NED to?