> something tells me that it won't be long before we could hear about
> a Sam460Sx ...

I very much doubt that we'll ever hear of such a thing for the 460SX lacks an FPU, as well as the 460GTx. While these two CPUs reaching up to 1.4 GHz are based on the PPC460 (or rather PPC464) core, the two FPU featuring CPUs 460GT (lacking USB and SATA) and 460EX reaching up to 1.0 GHz are still based on the PPC440 core. In conclusion, I predict no more than a Sam460ex @ 1.0 GHz to be the next Sam, if AMCC based at all.
The Linux kernel added support for 460EX/460GT with 2.6.26 (July 2008).