Halo 3 melee has now been "fixed" after careful testing by Bungie and pro-gamers from the MLG.
Boot up Halo and you'll be auto-updated. Two notes from Bungie on the patch:
NOTE: You may experience a few laggier-than-usual games after the update is installed as it resets your hosting and network connection history. This history should grow rapidly for an improved situation.
NOTE 2: If you run into a map loading error after the autoupdate, where a map freezes at a certain percentage loaded, try restarting Halo 3 while holding down the “A” button and "Up" on the D-Pad simultaneously while the game is booting. That may help with your problem. If the problem persists after this step, try following the normal steps for cleaning your disc or disc tray.
The melee fix can be summed up by this:
"Melee contests will only produce a winner if he has a sizeable health advantage over the loser. Otherwise, both players may die in the clash."
For a lengthy essay on the fix, head on over to Bungie.net.