As we mentioned last month, changes to negative keywords were starting to roll out to all advertisers.  As of now, those changes are complete!  Negative keywords uploaded at the campaign and ad group level will now be combined to filter traffic, working in tandem with one another. In the past, negative keywords at the ad group level would override those at the campaign level.

You'll find more detail on this change over on the adCenter Blog, you'll find more information for API users over on MSDN.