The current batch of overclocked GeForce 8800 GTS 512 graphics cards may still be sold but once the inventory runs out graphics card makers will only be allowed to sell 8800 GTS 512MB graphics cards with standard clockspeeds.
The source, who talked with someone from NVIDIA, doesn't know why NVIDIA is suddenly forbidding manufacturers to overclock but most likely it's because these overclocked cards are cannibalising the sales of the GeForce 8800 GTX and Ultra graphics cards. Both GPUs will be phased out in January 2008 but NVIDIA is having troubles to clear their inventory as there's little demand for these expensive GPUs.
The first batch of 8800 GTS 512 GPUs has almost entirely been shipped out and the next batch of GeForce 8800 GTS 512 GPUs is expected in mid-January or early February 2008. The source also heard NVIDIA is in talks with several graphics card vendors to launch a special edition of the 8800 which should clear some of NVIDIA's GTS 320/640 or GTX graphics cards inventory. This card may be available in mid-February and will be similar to the MSI GeForce 7900 GTO graphics card, it should be a bit faster than the GT but slower than the GTS.
In February we can also expect the first GeForce 9 seires graphics card, the GeForce 9600 GT (G94). The 9600 GT will be a 8" single slot card with 512MB DDR3 memory and it should score between the GeForce 8800 GT and GTS graphics card. For the next high-end NVIDIA graphics card you'll have to wait for the G100 which will ship in March.