Does this mean that if my characters are in a cave deep underground, without an exit from that cave for miles in either direction, that they can see in low-light conditions within the cave as long as there is a single star in the sky outside?
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Short answer: No.
Longer answer: That's up to the GM. If the GM sees it as some magical ability, much like werewolves and the moon, sure. However, more likely it was suggesting that as long as there is some light available, they can see. In my game, darkvision and infravision let you see in the dark. Period. But I like to keep things simple.
Compare the descriptions on page 77 of the Glossary (which I think is incredibly useful) of Darkvision and Infravision. Then compare the Dwarf with Elf racial abilities. That should give you lots of good guidance.