Researching this forum, most replies mention 0.3Vcc for LOW (logic 0) and 0.6Vcc for HIGH (logic 1). Here's the results of some actual measurements on an UNO Rev3:
As the input voltage increases from 0V, the input is read as 0 and switches to 1 at 2.60V. As the input voltage decreases from 5V, the input is read as 1 and switches to 0 at 2.35V. Hysteresis is 0.25V.
Expressed in terms of Vcc:
Input low voltage 0.47 Vcc Hysteresis 0.05 Vcc Input high voltage 0.52 Vcc