Well you have to look at the comparison between the two characters. War Machine is CRUCIAL to the iron-man plot, whereas Portman...not so much. What is she the world of Thor, the love interest of Thor scientist who tried to re-open that bridge, in spite of Thor finding alternative means.
I assume all of that will be further explained and portrayed in detail regarding the IM3 storyline, as opposed to a few lines of dialogue thrown in.
Same thing happened in TDKR with the Joker, no mention, no flashback. Only in this case it was out of respect for Ledger.