Here is a sketch that seems to work with an ATmega328P-MU. I get around 365mV, which would be about 75°C according to the example in the datasheet. Room temp is currently around 26°C, so that seems a bit high, but it's not a precise thing. Can the internal chip temp be that hot? The package certainly isn't that hot. It does respond fairly quickly (increases) when I put my finger on it. So maybe it's just not well-calibrated.
I thought maybe this could be coded just with standard Arduino function calls, but I don't think so; I ended up manipulating the registers directly. Something similar should work with the ATtinies, but the register definitions are not identical, so be sure to review them.
#include <Streaming.h> //http://arduiniana.org/libraries/streaming/
ADMUX = 0xC8; //channel 8
ADCSRA = 0xC7; //start the conversion
while (ADCSRA & _BV(ADSC)); //wait for it to complete
v = 1.1 * ADC / 1024.;
Serial << _FLOAT(v, 3) << endl;