The replacement for the purpose property in iOS 6 is a new Info.plist key named NSLocationUsageDescription (aka “Privacy – Location Usage Description”).
The key is documented in the Information Property List Key Reference but unfortunately it’s not mentioned with the deprecation note of the purpose property.
However, the CLLocationManager.h does have this comment:
*Deprecated. Set the purpose string in Info.plist using key NSLocationUsageDescription.
In your code, you could set both the key and the purpose property (but you may want to check if the location manager responds to that selector first if/when that method is actually removed in the future).
If running under iOS 6, the location manager will use the key.
When running under less than iOS 6, the key will be ignored and the purpose property will be used.