Thanks guys, I experimented a bit more after I posted, and I also found that getting behind them is the key. Or letting them get close and tagging their head repeatedly while backing up.
For anyone who didn't get my little joke in the topic, 101_proof is correct, I'm talking about Halo. I got it last night, and have wasted most of today playing it.
As to the claim of it being the best game ever....I disagree. It is fun as hell, and it is likely the best 1st person shooter ever, but I've got several complaints. The biggest of which is the lack of interactive environment.
The title of best game ever still rests with Grand Theft Auto III & Vice City, hands down.