We recently announced  several changes and improvements to the way we report AdWords impression share  (IS). We are now making these changes available through the v201209 version of AdWords API reports as well.
Around the second week of December, the following new columns will be available for account , campaign  and adgroup  performance reports:
  ContentImpressionShare 
  ContentBudgetLostImpressionShare 
  ContentRankLostImpressionShare 
  SearchBudgetLostImpressionShare 
  SearchExactMatchImpressionShare 
  SearchImpressionShare 
  SearchRankLostImpressionShare 
 
We now provide distinct search and display columns, so you can clearly separate search and display impression share. We also support “Hour of day” segmentation to evaluate how your ad coverage varies by the hour. You can get hourly segmentation of impression share data by requesting the HourOfDay column in your reports.
Finally, we’re improving the accuracy of how we calculate impression share data. As previously mentioned on our product blog  and via social media , historical IS reporting data is now only available going back to October 1, 2012.
Phasing out old IS columns in February 2013 
With the new IS columns now available, we’re planning to phase out the old IS columns (BudgetLostImpressionShare, ExactMatchImpressionShare, ImpressionShare and 
QualityLostImpressionShare) in February 2013.
If you face any issues with the above changes or have any questions about the AdWords API, you can ask us on our developer forum , or at the upcoming live office hours  with the Developer Relations team.
  - Anash P. Oommen , AdWords API Team