The A8N-E/SLI/Premium mobos do not have built-in temp monitoring for the NF4; the mobo temp you get from software programs or BIOS is from a location somwhere near the NB I believe, but I am not certain where exactly. It is almost certainly not the MOSFET temp, as I have a probe in the MOSFET heatsink that reads 44C at idle and 47C under load (according to temp probe hooked up to an Aerogate III), while my mobo temp rarely hits 40C under full load.
Those who have measured the chipset block with infrared temperarture probes, or the flat sensors hooked up to a temp monitor generally report 50-65C. The NF4s are rated for up to 90C, I believe, and even ASUS claims that the A8N Premium heatpipe keeps the NF4 at 70C (under full load, I assume). However, there are scattered claims that chipset temps exceeding 65C might lead to system instability; the validity of such claims is questionable, but certainly not inconceivable, as maximum safe temperature and maximum stable temperature are not necessarily interchangeable terms.
The only time I would be concerned about the sufficiency of the heatpipe to keep the NF4 cool is if you had extraordinarily poor airflow within the case, or were running the system overclocked in very high ambient temperartures. There are also reports that the heatpipe does not take well to inversion (i.e. a mobo mounted in some of the recent Lian Li cases).
