Of course, the rumors about this have been floating around since the 7800GTX's launch. It'd certainly be an expected move to release a speed-ramped chip at the same time that R520 is officially announced/released, & I'm almost sure that NVIDIA has had it planned for a while now.

"NVIDIA’s ForceWare 80.40 drivers, which were recently leaked on the Internet and are available at 3DChipset web-site, list G72 and NV48 graphics processing units, which were not yet released, among all the graphics chips the driver is capable of support. The driver also supports yet unreleased, but already somewhat known – GeForce 6600 LE, nForce4 C51 IGP, G70GL and Quadro FX 4500 – products."