Acoustic app Thermometer 147 measures the temperature of the air near the microphone of your phone.
The application estimates temperature by interpretation. The iPhone does not contain an actual thermometer and results not guaranteed accurate.
Precision 3.6°F/2°C
It does this by sending out a waveform through the speaker.
Only tested on iPhone 4.
app-sources.com
crowdt.com
Download it and enjoy!