Block module will disable all block caching when one or more modules implement hook_node_grant(). The problem is that site builders can consider their blocks are safe, even when using node access modules. This cause serious troubles on sites using a lot of blocks.
I propose to add a simple variable which allows site builders to force block cache when using node access module for people that needs block caching deseperatly and are willing to set their own block caching policy using the hook_block_info_alter().