After having some spare time on my hands I redesigned my PI smart power supply switch and reduce the component count considerably.
I've also added a power down switch not that it's really a requirement but I guess if you're too lazy to pull the plug this might come in handy.
Killing the power should only be done as a last resort since it increases the possibility of a corrupted / unbootable file system.
While other relay circuits already exist as mentioned in this thread they cannot handle situations such as a system reboot because the UART TX output goes low for a split second.
When the relay is off 5v travels down the relay switch via a 22k resistor to charge the 220uF capacitor.
Once there is sufficient voltage this turns on transistors Tr1 and Tr2 (Darlington Pair) to power up the relay.
When the Power On switch makes contact the relay circuit fires up instantly and the capacitor gradually discharges.
By then the voltage output from the GIPO UART TX pin should take over the role of keeping the capacitor charged up.
If however the GIPO UART TX goes low i.e. a power down then the capacitor would discharge and eventual the relay shuts down.