NVIDIA has obviously not moved past it's renaming schemes. The 580 being a 480 but re-tweaked with 512 shaders and increased clock speeds with slight tweaks to power consumption, but still the same hardware (2 monitor output limitations, etc still apply).
Now the mobile GPU range hits with the 500M range. The 540M chip maintains the same 96 CUDA cores and 128-bit memory interface as the GT 435M but earns it's "next-generation" name by increase clock speeds.
Graphics clock speed is now 672MHz and processor clock speeds set at 1244MHz - while memory clocks sit at 900MHz. Power requirements remain unchanged. China gets the GT 540M immediately from an Acer product where the rest of the world has to wait for the 'new' 540M sometime in the next month.