All what MartinM57 said, plus, when switching 0V one has to consider what happens to signals which are between the switched device and PICAXE when it's off. For example if there were a serial out from the PICAXE to iPAQ it could be holding the iPAQ to 0V via its serial in. Similar problems can occur when switching +V if signals into it were high.
In this case, switching iPAQ 0V could cause pin3 to float up to +V and the PICAXE jam up expecting serial, and while doing that never timeout and turn iPAQ power back on. Putting a pull-down on pin3 to stop that could allow some current to still flow through the iPAQ.
It's quite complicated sometimes to switch a component off in the middle of an otherwise powered system, most microprocessors with internal ESD diode clamps are prone to suffering from 'phantom power' effects where they can still keep running even with +V disconnected.
One thing from the above, is that it's probably best to disable interrupts when power is off and re-enable them when turned back on.
In this case, switching iPAQ 0V could cause pin3 to float up to +V and the PICAXE jam up expecting serial, and while doing that never timeout and turn iPAQ power back on. Putting a pull-down on pin3 to stop that could allow some current to still flow through the iPAQ.
It's quite complicated sometimes to switch a component off in the middle of an otherwise powered system, most microprocessors with internal ESD diode clamps are prone to suffering from 'phantom power' effects where they can still keep running even with +V disconnected.
One thing from the above, is that it's probably best to disable interrupts when power is off and re-enable them when turned back on.