I spent a year working for Noam Sobel, a researcher in all things smell related. I worked on various parts of an electronic nose project which used sensor from Nanomix to build a device which could "learn" about odors and then identify them in the future. I'm not going to get into the technical side of how the sensors work, but you can check out Nanomix's website for more information.
The device is based on a 40MIPS Microchip dsPIC33. A pain in the ass to learn how to use, but it is so feature rich and does everything for a really low price. The A2D is a 1MSPS AD7329 with a built in 8:1 mux. Each IC has 10 sensors which connect to a 16:1 mux, then to a high-end op amp configured as a transimpedance amplifier. This then connects to the 8:1 A2D mux. The sensors are driven with the output of a 12bit D2A (AD667JN) with a buffered output. Finally, data is transfered via an FTDI USB FIFO. You can find a schematic of it here.
The circuit is designed sweep these nanotube sensors with a triangle wave while measuring the resistance through the sensors. It has 90 sensor inputs (plus 30 other redundant inputs) that it samples at a fairly high speed and dumps the data on the USB bus. It actually works pretty well. It’s also got a hookup for a temperature/humidity sensor as well.