Don't understand what you mean by using your dad's PDA to surf
the net?
If you mean using his PDA to surf the net using Wi-Fi, which Wi-Fi
Access Point (AP) are you connecting to? If it is those free one
like [email protected], Skynetgolbal at McDonald, etc., or those you set
up at home using wireless router, then of course it it not illegal.
If you use your neighbour's or other private AP that is open, then
it is illegal.
However, if you are using his PDA to surf the net using 3G/GPRS,
and it is paid by your dad's company (Singtel), of course it is not
illegal in that sense. But it depends on your dad's company
acceptable use policy.