There is some example code here for getting a temperature from a thermistor, in Project 13, here: http://www.arduinoevilgenius.com/projects
. Go to the download page to get all the project sketches in a zip.
This is the crucial bit:
long a = analogRead(analogPin);
float temp = beta / (log(((1025.0 * resistance / a) - resistance) / resistance) + (beta / 298.0)) - 273.0;
Although, now I come to look at it again, it looks like it should simplify.
Anyway, it works, and the logger writes the temps to EEPROM and then on a command from the serial monitor it spews them all back up again.