The same dumb joke could have been (and probably was) used when Kerry crashed his bike. Much ado about very little.
The difference is who makes the crack. You would not hear Bush saying that to reporters upon hearing about a Kerry mishap, nor would you hear the press then chortle along *at* the target of their mirth (as I am confident happened here. Look at that idiot Bush, they all laughed knowingly together).
See?
For one who is running for president to make such a "joke" is childish, petty, snotty and graceless.