All of my entites extend an abstract class Model to inherit the 'id' property and some other functionality. It has the @MappedSuperclass annotation.
If the id property is declared private to the Model class, I'll get the following exception when using entites extending it:
[03-Mar-2010 11:36:29] exception 'ReflectionException' with message 'Property id does not exist' in library/Doctrine/ORM/Mapping/ClassMetadata.php:370
#0 library/Doctrine/ORM/Mapping/ClassMetadata.php(370): ReflectionClass->getProperty('id')
#1 [internal function]: Doctrine\ORM\Mapping\ClassMetadata->__wakeup()
I haven't looked at Doctrine code, but perhaps you should be looking for the property on the class which actually has the @Column annotation?