As far as the IMC is concerned, you will learn to fly on whatever *certified* instruments there are in the plane.
AFAIK, a portable device can never be certified and thus cannot officially be used for navigation under IFR.
I am not sure that is quite true.
In general, in private flying, nobody prescribes what navigation method should be
used.
Only equipment to be
carried is prescribed; this to me implies it should be functional but it cannot possibly imply it should actually be used because e.g. the UK ADF for all IFR in CAS requirement cannot possibly be a requirement to navigate enroute using the ADF...
So a handheld GPS should be 100% legal for navigation in IMC.
Enroute (Eurocontrol flight plan) IFR is a different thing because there you need to
carry a BRNAV approved GPS installation, and that can only be panel mounted. But even there there is no law which says what device you actually navigate with.
This stuff has been done to death many years ago; if you google Usenet for "tuna sandwich navigation" you will find long threads there
BTW the built in Ipad2 GPS is rubbish. I have flown with mine and it is quite capable of losing the GPS fix anytime during a flight. You need an external GPS.