Not true for the AGP 256MB DDR3 cards.
However the 6800GS pci-e and the new gainward 6800GS 512MB cards are limited to 12 pipes
The 6800GS AGP is the same card design as the 6800GT with the same NV40 chip, it's hit and miss whether it'll unlock or not but it seems the majority do.
I got a b-grade 6800GS from overclockers the other day (figured out why too, it was missing the thermal pads for the voltage regulator heatsink). Soon as I got it, I tried unlocking it to 16/6 and that worked great, then I overclocked it an it went up to a little over 400/1100

Now with a quick change of the card's BIOS, it's locked at 16/6 and 400/1100
Certainly compares well with the 6800GT now