The Turkish government has decided to change the DST schedule for 2011, pushing back the start-date from March 27th to March 28th. The existing timezone (UTC +2:00) Athens, Bucharest, Istanbul is shared with several countries who are not changing their DST schedule. Therefore a new timezone has been added, Turkey Standard Time.
A Windows hotfix has been published to enable this http://support.microsoft.com/KB/2523087 . Customers who want the update should install the hotfix and select the new timezone, display name (UTC +2:00) Istanbul.