and then there's my ex... would it be at all awkward if I gave her something? Would her new boyfriend kick my arse, would she eat it and vomit the pieces into my eye? (I really was a bastard to her and I still feel horrible for it. Thank God I've learned my lesson)
If you're still friends, it might be okay, especially if you're both part of a social group where everybody exchanges gifts. Just don't make it anything too personal or expensive.
But, otherwise, yes, it will be will be awkward; personally, I would skip it.
BTW, if you're looking at this possible Christmas gift as a substitute for an apology you never made, it might be better to be direct: send her a Christmas card (either e- or paper) w/ a note of apology.