How can I sense very high temperatures with Arduino?
You probably want to use a thermocouple. It'll give you the ability to measure much higher temps than regular temperature sensor. You'll also need a IC like the MAX6675 to interface with it.


  The MAX6675 performs cold-junction compensation and digitizes the
  signal from a type-K thermocouple. The data is output in a 12-bit
  resolution, SPI™-compatible, read-only format.
  
  This converter resolves temperatures to 0.25°C, allows readings as
  high as +1024°C, and exhibits thermocouple accuracy of 8 LSBs for
  temperatures ranging from 0°C to +700°C.




Page 6 of the datasheet has the serial interface protocol information and. You can really just set the Chip Select line and it'll give you 16 bits of information, 12 of which will be the temperature.