Sometime in Drupal 5 or earlier, we moved the user table's access column to a separate table because it was updated every page request. In Drupal 6 and later, this only happens every 3 minutes per user, overwritable with a number of seconds in
session_write_interval. I think we should not carry this hack forward to Drupal 7 since core has a different solution. It would be good to deploy this on Drupal 6, so we can verify it scales, and have one less thing changing in production on launch day.