You guys are all wrong. 1.51 does not check for security on Wave 3 games. 1.6 prevents you from loading an unsafe game, that's a feature not a glitch. So, if you want to play these Wave 4 games, use activate.iso instead of downgrading to an unsafe firmware!
Or wait until 1.7, which is the smart choice.
If you play these Wave 4 games, either downgrading or using activate.iso, you are NOT safe from the "banhammer". When you put a disc in your 360 that is not stealthed correctly , 1.6 blocks it, with activate.iso you are telling it to not care, and boot anyways! 1.51 boots no matter what for Wave 3 and 4, meaning if you get banned, don't blame iXtreme, blame yourselves.