I reported this morning about Google resetting many of the "inactive" keywords in a Google AdWords account to the "active" status. Keywords become inactive often if the "quality" of those ads are too low, most likely because the click-through rate on that keyword is too low. It seems as if Google has reset many of the "inactive" keywords in AdWords advertisers campaigns, so they can start fresh.
Postscript From Danny: This is likely related to what Google announced last month, that for some terms, it would be pushing ads out more often. See Google AdWords Broad Match To Act Differently For Commercial, Non-Commercial Terms for more.