Many temperature sensors are available in the standard transistor TO-92 package. Are these designed only to measure air temperature? Say I had a sheet of metal, would I just use some heatsink compound to mount the sensor, and it will measure the temperature of the metal accurately?

 

Of course. They are made to output the ambient temperature. Attaching the package to a surface will make it respond to that temperature. For better accuracy, place some foam over the LM34(35) to isolate it from the air.

With good insulation on the leads, they work fine in liquids, too.

Nothing beats a really weenie thermistor for fast response, but you have to linearize the output.

 

If you want the sensor to stay permanently attached, use a bit of J-B Weld (the regular, not the 5-minute stuff) and glue the flat face to the surface of your material.

For the insulation, consider using this stuff:
http://www.homedepot.com/h_d1/N-5yc...splay?langId=-1&storeId=10051&catalogId=10053
It's low expansion, so it won't try to rip the leads off your temp sensor. It's flexible when it cures. Don't use the high expansion foam.