In Dragon Ball Z it is mentioned that Androids dont have ki. If I recall correctly, when 17 is fighting Piccolo one of the Z fighters says Piccolo must be figthing an Android because he can't sense his ki. But later it's been said that 17 and 18 are modified humans, now we know that 17 can raise his power level by doing some kind of exercises, and we see he has an aura when fighting with Goku. Does it mean he has ki? What about also when he gave his energy in Dragon Ball Z for Goku to make a Genki Dama against Buu?
When the Androids first appear, they are operating almost entirely on their robotic side, and denying their human characteristics. That might have something to do with why there was no Ki to be sensed originally. However, they must have Ki, based on the fact that the androids can give birth, which means they are alive, and we know all living things have ki, even the animals and plants.
Ok, this question have been answered in the last episode (121), Android 17 states he and 18 dont have ki, in spite of he can have an aura when fighting.