Engineers at Brown University have been working on a device using nanotechnology that could be used to measure blood glucose concentrations in human saliva. According to the article, glucose concentrations in saliva are about 100 times lower than in blood. The greater blood concentration is easier to measure but since they are so much lower in saliva it is much more difficult to measure accurately.
Their research is in the early stages and if it proves to be accurate and cost effective, perhaps one day we will be able to end the painful finger sticks. Here's hoping!