I don't think this is news to anyone who overclocks, or if it is, they shouldn't be overclocking.  You really need to watch ALL of your available temperatures.  The problems happen when even one exceeds the limits.  The sensor is invariably some distance away from the heat source even so it's reading will be a bit cooler (usually only a couple degC)

Many stock cards have very poor VRM cooling, or poor cooling in general.  I just got a couple 6870 Sapphires (never owned a sapphire, nor a 6870) and the VRM heatsinks are way too small to be doing overclocking.  The primary HSF looks decent though.  Untested since I'm still waiting on the motherboard to show up.