Would the soldiers shot at Ft. Hood be eligible for the Purple Heart?

They were after all on active duty, though I don’t know if you could say they were in the line of duty.

Probably not.

From the wikipedia entry:

The attack did not come from “an enemy of the United States,” or “an opposing armed force,” or a “hostile foreign force,” or an “international terrorist attack” (unless the Secretary of the Army finds otherwise), nor were the injured and killed troops serving outside the U.S. as part of a peacekeeping force.

I agree - probably not.This was a crime, not an act of war.

It’s worth noting that the (admittedly early) reports now say the perp was in the same congregation as (and thus theoretically in potential contact with) some of the 9-11 hijackers, and has spoken positively about violent jihad for years. It is possible that this will be counted as an externally-influenced attack. Whether that’s sufficient for a Purple Heart is beyond my ability to guess.