Seeing this thread was an interesting event for me. In 2001, I designed a system that monitors temperatures for use in supermarkets, relays the temperature data over a wireless link to a base station, and interfaces to an embedded PC. It is still in production as of this posting, and I am starting work on the next generation of the product.
The actual hardware design is fairly simple. The temperature sensor is a thermistor and is tied to a high precision pull-up resistor. The thermistor's resistance can thus be converted to a voltage that can be sampled by a microprocessor equipped with a decent A/D converter. A very cheap micro that does this is the Texas Instrument MSP430F1222. The A/D is 12-bits which allows temperature resolution. By knowing the resistance/temperature curve of the the thermistor, the A/D counts can be converted to engineering units (in this case, temperature in degrees C or degrees F). This data can then be output from the processor's UART or "bit banged" as a serial data stream into a transmitter.
For the transmitter, there are lots of choices. A simple and cheap one is made by Linx Technologies, the TX-433-LC. A simple T-network of three resistors, and a cheap antenna can provide data ranges up of to 60-meters with the matching receiver while meeting FCC requirements. Add a plastic housing , a battery to power the device, wiring for the temperature sensors, and some connectors; poof the sensor is done for about $20.
The receiver is a bit simpler since all one needs is the matching receiver chip and a microprocessor that takes the serial output from the receiver chip and outputs it to a UART in RS-232 format.