On October 11, Bungie had a Halo: Reach Challenge to kill 10 enemies on the level Exodus on Normal while jet-packing.  It wasn't that difficult, but there was something that came to light during the process.

Follow up:

For a kill to count while jet-packing, the thrust button must be held.  If you thrust then let go and make a kill, it doesn't count.  If you are free-falling from thrusting and make a kill, it doesn't count.  The kills count, just not as kills while jet-packing.
You must be holding the thrust button when you make the kill to get the medal and for the kill to count as a jet-packing kill.
When you think about it, it makes sense.  If you were to jump and make a kill, that's not jet-packing, just like if you were to be falling and make a kill.
So now you know; to get the credit for a jet-packing kill, you must be holding the button when the kill occurs.

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