The '51 can NOT read the temperature, it can read and process the output from a temperature sensor. Sensors are available with digital output (such as the Dalls chip mentioned by Charles) or analog output (RTC, thermistors etc). If you chose analog output you will need to use a chip with a built in analog to digital converter or use an external AtoD. The REAL advantage to to using a microcontroller instead of a dumb thermoswitch lies in the fact that you can control hysteresis and if using a triac or SCR even go for PID control of your heater.

Erik