My girlfriend is currently in Spain. She has an Iphone 4 with the Data plan set up for her contract (No wi-fi in the villa shes staying in). Will she be able to use Pingchat to chat to me without incurring any additional charges? or will o2 class this as a roaming charge on the data plan?

If she is using ping chat abroad she will use data and be charged a fortune! If no wifi don't use!

As the other poster has said, ping chat and similar apps work by sending data over the network rather than using SMS or MMS. She would be charged at data roaming rates until she reached the cap and then data would be cut off.

Some networks do offer international data bolt ons and it may be worth contacting O2 to enquire about this

If she's on O2 it's charged at either £2 or £3 per mb (I can't remember off the top of my head). If she has data roaming switched on, she'll get a text at the beginning of the trip telling her how much per MB she'll be charged and that they will text her when her data spend reaches £20, and again when it gets to about £40 (I think you get a total of 50 MB for your forty quid). After that it'll stop using data and won't charge you any more within that billing month. If your trip falls over a billing date, the allowance is reset and you'll get another £40-worth (so a total of £80).

It's still very expensive though.