If I understand them correctly, a service dependency can prevent a dependent service being checked and/or sending notifications when the service it depends on is failing. But if a dependent service is checked and fails before the service it depends on checks and fails then they will both appear as down in the status.cgi. I know there are service filters for SERVICE_CHECKS_ENABLED and SERVICE_NOTIFICATIONS_ENABLED but it seems that when the service they are depending on fails, the dependent services do not show as having checks or notifications disabled and don't seem to be filtered by my serviceprops value of 42 (Not In Scheduled Downtime & Has Not Been Acknowledged & Active Checks Enabled).
Hope that makes sense. Basically if I have a hostgroup where things like Disk Usage is monitored by SNMP, when I look at status.cgi for problems if SNMP is failing I don't want to see all the failing dependent services as well, such as Disk Usage.