I agree with ptp. My daughter was physically unable to apologise until recently (she just couldn't get the words out), she was so sensitive to our disapproval of whatever the action was that she was completely overwhelmed (and she didn't always understand why it would be necessary). We turned it on its head much as Potatoes has suggested. More recently (she's now 5.5)she seems ok with it and we've started talking in more detail about why an action might warrant an apology (and not just about her own actions, but characters in books and so on). She now apologises quite readily (though she rarely has cause to).
DD's preschool teacher once said to me that she never forces kids to apologise to each other because it takes all the meaning away from it (particularly if your forced to apologise and you don't understand why and/or genuinely don't feel you should have to). Instead she felt it was much more important for adults to model appropriate apologies.
I think too that that age is hard for gifted kids. Their asynchrony is so great - with heightened awareness but very limited understanding or experience re their emotional responses.
Best of luck!
