PowerGlove sent me a nice patch for this which I just added:

Previously, if you didn't check the "Show others your online status" option, it would still be pretty obvious that you were online because your "Last Active" time would still constantly update to be ~now. This has now been changed so that if you were last active within the last 72 hours, then the exact time will not be displayed (except for admins), and it will instead just say "(Recently)".

If you would like people to be able to more exactly tell when you were online, then you can of course just enable the "show others your online status" option in your profile settings.