Stephen Toulouse, Director of Policy Enforcement on Xbox Live, let people know via his Twitter account that they would find out somehow:
"I’ve been asked about unauthorized pre-release play on Halo: Reach. Let me address that... as with all unauthorized play on Xbox LIVE, anyone playing any unauthorized title runs the risk of account permaban and console... remember, legit store bought copy runs no risk of ban. Key word: UNAUTHORIZED. We have ways of knowing. :>"This is no surprise, as this is the same tact that Microsoft takes with anyone that pirates games; the moment they connect to Live, their play habits are transmitted to Microsoft. While hardcore pirates already have ways of getting around these bans such as separate consoles for pirated play, this should be enough to scare off the average punter.