For various reasons, LEGO only sells the temperature sensor through it's educational channel. That's fine... but how can the average user, with only the Retail software, use them? That's been a problem I've had a lot of folks email about, asking how I use it under NXT-G and if I can "share my secret".
Well, how about Steve Hassenplug just hosts my secrets? Look about half-way down this page, at the "Temperature Block" and the "I2C blocks" below it
Inside the LEGO temperature sensor is a Texas Instruments chip (a TMP275 chip to be exact), which can be controlled like any other I2C device. That's handy, and there are 3rd party add-on blocks that will let you let you read a byte via I2C, or write a byte via I2C, or even read a series of bytes (multi-byte read) via I2C. Steve has also been kind enough to host copies of these right under the My Block I drew up to read the sensor. Note that you need to configure the sensor before reading it - I normally configure it for highest resolution. To see how this block works (and how to configure it), take a look at the program fragments above, from my Brickshelf gallery.
Please let me know how well this works, and if there are any problems I need to address. You will need to have both of the I2C blocks installed into NXT-G, as well as have the My Block in your program, and configure the sensor correctly with a single I2C write... but it should make it a little easier for Retail set users to enjoy this fun sensor.