From here:
http://www.ladyada.net/learn/sensors/tmp36.html
Problems you may encounter with multiple sensors...
If, when adding more sensors, you find that the temperature is inconsistant, this indicates that the sensors are interfering with each other when switching the analog reading circuit from one pin to the other. You can fix this by doing two delayed readings and tossing out the first one
The link:
A user in the customer forums had a problem with multiplexing analog signals, they seemed to be very noisy and inconsistent. Turns out it was because the sensors are high impedance. This means that if there is capacitance on the ADC line or on any sort of sample&hold capacitor, it may take a while for it to charge up. Read these threads for some great analysis! View here and here?
Hope this helps...