Google's AdWords Keyword Tool has been experiencing some substantial and ongoing issues over the past week. Complaints have arisen from marketers using the service that the results are showing zero search volume when performing exact match requests.
Issues with the popular Google service were first reported on Thanksgiving. It seems that the zero results aren't showing in all searches but are for a random selection of search results. Preliminary tests performed reported zero search volumes for both global and local searches, even when the country location was changed.
It seems that most head and middle-tail related keyword searched are being affected. It is premature to state if long-tail phrase searches have been affected as well due to the normally low search results associated with these types of phrases.
The issue doesn't seem to follow any particular keyword or industry type, or to match any definable criteria making it difficult to analyze the results being given.
Although the numbers may be incorrect and/or non-existent, the data is of some value. If you do manage to get results, you can use comparative analysis to make use of the limited data.
For instance, if you perform a search for [apples] that yields 6,000 results, and a search for [bananas] that yields 3000 results, you can be relatively assured that twice as many people are searching for one over the other assuming all else is equal such as significant ranking report runs and other automated queries for one phrase over the other, etc.