I plugged it into my power sucking tablet (Galaxy 10.1 2) and boy, it sure does get warm. The FAQ says this is OK, and "maximum temperature for the mintyboost chip is over 100 degrees C". Mine clocks in at about 84C when charging the Galaxy. I looked up the LT1302 data sheet and under "Absolute Maximum Ratings" (p. 2) it lists the operating temperature range as maxing out at 70C.
[edit...]
OK, reading more closely, it looks like the 70C in the maximum ratings is referring to ambient temperature; i.e., the chip might not work if left on your car's dashboard on a hot sunny day. The maximum chip temp ("Tjmax" in the fine print to the right) indicates it can go to 125C, well above boiling water.
Page 12 goes into more details, in particular:
So it looks like there's no risk of the chip cooking itself, but the charging rate might slow down a bit if it starts thermal cycling.The LT1302 contains a thermal shutdown feature which
protects against excessive internal (junction) tempera-
ture. If the junction temperature of the device exceeds the
protection threshold, the device will begin cycling be-
tween normal operation and an off state. The cycling is not
harmful to the part. The thermal cycling occurs at a slow
rate, typically 10ms to several seconds, which depends on
the power dissipation and the thermal time constants of
the package and heat sinking.