What's the difference between
_getCacheId is not enough to cache a block?
getCacheKeyInfo is used to determine the cache key if one is not set directly for blocks.
It is used in
Mage_Core_Block_Abstract::getCacheKey() that is used
_loadCache method on the same block
_getCacheId is not used at all for block caching and it is deprecated starting
Honestly I have no idea what was the use if it before 1.4.
126.96.36.199-alpha, magento deprecated
_getCacheId() may be in order to improve
You see more details about deprecated from 1.5.0 and check this link at http://docs.databyte.at/magento/community/1.5/deprecated.html
getCacheKeyInfo() use for cache