This just in from GossipCop:
"Many of you asked whether Kristen Stewart will be part of the “Hope for Haiti” event. Gossip Cop has learned that Stewart would love to be involved, but will be occupied at the Sundance Film Festival and unable to participate."
Scheduling conflicts happen. It's good to know that she would have been down for participating. Still no word on Taylor or any of the other Twilight cast members attending. Once I get information, I will pass it along.