Vinny Lingham reports finding a new metric on his Google AdWords console named Impression Share. Impression Share is a statistic available at both the campaign and account level, which shows the share of all impressions an ad earned against others that are competing for those. From the help pages:
Campaign: If an advertiser has more than one campaign competing in the same target market, the resulting impression share for any one campaign won’t be penalized if it loses an impression opportunity to another campaign from within the same account.
Account: Reports at this level will not penalize your impression share from the competition across all your campaigns.