I am reporting some findings that I hope will help others who decide to try any access control module currently available to Drupal 5.1 or earlier.
There is currently an issue in the 5.1 build of Drupal which will cause a site with high numbers of nodes (4000 in my case) to error out when rebuilding node access permissions or when enabling / disabling a node access control module. This error is due to node_access_rebuild() calling node_load() and loading every node into cache thus causing an out of memory error. This occurs not only when using the rebuild permissions function in Content Management -> Post Settings but also anytime the node_access_rebuild() function is called such as when enabling or disabling access control modules.
The patch for this issue can be found here: http://drupal.org/node/108752 , or users can update to the current HEAD version of 5.x for the fix. There are plans to further address the issue in 6.x via a batching process.
It would be nice if module developers would include some information in the module documentation specifically about this issue. I believe after reading many different memory error issues throughout DO, that this is a big contributor to many mystery error issues.
Thanks