If Bush hadn't said 'this is from Psalm 23' would you have known any different? What would your reaction be then?
It was an appropriate 'phrase' for the speech.
Remember, Bush said for 'YOU' are with me. He didn't say God, or Jesus, or anything. 'YOU' could mean anything that gives us comfort... for the majority of Americans who are religious in some way, whether Hindu, Christian, Jewish, or Moslem, that means a belief in God/Jesus/Allah etc. For those of you who are not religious, it could mean family or friends.
What the heck is wrong with that?
I hate it when people jump all over something just because it happens to be based on a Biblical teaching. If it's appropriate, then who the heck cares?