At least according to x-bit sources; this would sound more logical if the NV30 would be nearing widespread release however we won't see it selling everywhere until the beginning of next year. Obviously Ti4600s won't dissapear right away but will be fading away eventually as we approach year's end:

Our sources with knowledge of the matter told me that NVIDIA had stopped producing the GeForce4 Ti4600 graphics processors at TSMC fabs and the company's closest partners confirmed the information. Moreover, some retailers indicated the tight supply of the GeForce4 Ti4600 based products in the channel.

The lack of the GeForce4 Titanim processors was caused by the shift of the GeForce4 Ti4200 to AGP 8x specifications and end-of-life (EOL) for the GeForce4 Ti4600 GPUs.