Jeffrey Posnick, AdWords API Team posted an announcement in a Google Groups thread about budget optimizer changes that AdWords API users might need to know about. If you are an AdWords API user and you use Budget Optimizer, then you should read this message from Jeff.
Each Campaign object has a budgetOptimizerSettings property, which in turn has a takeOnOptimizedBids setting. Currently, if takeOnOptimizedBids is set to false and you turn off the Budget Optimizer in a campaign, the original bids that were manually set for the ad groups and criteria contained in the campaign will be estored. In other words, the AdWords API will save your old bids from before the Budget Optimizer was turned on, and restore them when it is turned off. If takeOnOptimizedBids is set to true, however, the old bids will not be restored when the Budget Optimizer is turned off. Instead, the last bids that were automatically set by the Budget Optimizer will persist for the ad groups and criteria in your campaign. You can, if you wish, use the AdWords API to manually update the bids for ad groups and criteria once the Budget Optimizer is turned off in either scenario.
The upcoming change will effectively treat takeOnOptimizedBids as if it were set to true (regardless of what it is actually set to) for all campaigns using the Budget Optimizer. Once the Budget Optimizer is turned off, all bids will be set to the last bid used by the Budget Optimizer. This change will match the behavior currently in place when the Budget Optimizer is turned off via the AdWords web interface.
We're planning on making this change to AdWords API behavior on May 19. It is not likely that you will have to make any substantial changes to your code in advance of this change, but please take the new behavior into account when you write code that turns the Budget Optimizer off.
I quoted Jeff's words exactly so that nothing gets lost in the translation.
Forum discussion at Google Groups.