Temperature-reading accessories for smartphones are not too new, but the Robocat take appears to be the most portable one yet-- the Thermodo, a small (3.5cm) thermometer one can plug into device headphone jacks.
The Thermodo carries a highly-sensitive sensor inside a tiny package, and works together with a companion app. Once plugged in iOS or Android devices it measures air temperatures, with the app providing readings and graphs.
The device is reportedly even a bit too sensitive-- touching it affects readings, and while the app provides some compensation adjustments the makers suggest using an audio extension cable for more accurate readings.