Microsoft announced they are adding a new ad serving feature to adCenter.
The new feature is ad rotation and will allow advertisers to either (1) rotate the ads by to optimize for clicks or (2) rotate ads more evenly despite clicks or conversions.
Here is a screen shot of the feature which is found under at the ad group level:
Google AdWords allows you to rotate ads but they offer three options, (1) optimize for clicks, (2) optimize for conversions or (3) rotate ads evenly. But Google tried to remove ad rotation in April but due to advertiser complaints they brought back the feature to those who want it.