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A Google AdWords Help thread has complaints from Google AdWords advertisers that the Google Keyword Tool does not show search volume when you do exact match requests.
Search Engine Land received confirmation of this bug from Google saying it is a known issue Google is working to resolve. Google told Search Engine Land:
We're aware of a potential issue that is affecting the keyword tool in AdWords, and are working hard to fix it.
SEO Consult blog has a picture of this in action.
Forum discussion at Google AdWords Help and WebmasterWorld.
