From the State Department's front page:
For missing family, call 1-888-407-4747. To help with relief efforts, text "HAITI" to "90999" and $10 will be given automatically to the Red Cross, charged to your cell phone bill.
See the State Department's front page for additional information re the Red Cross and Mercy Corps, too:
Thanks to my friend, Caryn Rose, for the tip.
[Postscript an hour later: Haiti's Prime Minister has told CNN "well over 100,000" have died: