Does deleting iCloud backups delete the associated data immediately or does Apple retain the data for a period of time?
I believe there would be a time frame to which the data would be able to be retained given some simple knowledge of server configurations.
You could think of the iCloud server as a series of hard drives all connected together and connected to the Internet. Some of the hard drive are used for saving your information and your iCloud account allows you to choose how big you want your iCloud storage capacity to be.
Other hard drives are used to backup those hard drives. These would be used in the case a large number of the hard drives were not working properly. In this case your information is still saved, which is a very smart and good business practice.
Some other hard drives are used when only one of the hard drives with customer information is failing, then this drive will pickup where it left off so there are no service issues. Also, very smart practice.
So as far as that information which has been deleted, it is probably still able to be retained for a period of time, after which it would be overwritten on the drives which backup the drives with the original data. Your privacy would still be respected due to the high encryption level of iCloud.
Lastly, retaining the information through an interface such as the Settings>iCloud pane in iOS would not be available. However, the information may still be able to be retained in the case it were done on accident.