According to a Twitter post, CNN is reporting it's Ted Kennedy.
@BreakingNewsOn: A senator has collapsed at the Congressional luncheon for President Barack Obama; CNN reports it is Sen. Ted Kennedy.
I'm trying to find other reports that confirm.
Update: CNN's website is confirming:
Sen. Edward Kennedy has collapsed at a luncheon honoring President Obama. Paramedics have responded to the scene.
Update 2: Fox is reporting that Kennedy and Sen. Robert Byrd were both removed by emergency medical personnel during President Obama's celebratory inaugural luncheon.