LionGames.com is offering a $20 discount off of all Halo 3 retail editions. The catch is you'll be paying the full discounted price upon purchasing. Lion Games does imply that this is a limited time offer, encouraging interested shoppers to 'buy now,' meaning you'll be forking over at least $40 for a product that's unlikely to ship for at least another nine months. The deal could certainly be a device for the newly launched retailer to build some quick capital; but if you don't mind contributing to that, then go ahead and purchase a copy (or two).
We can't vouch for Lion Games' credibility, but the site, headquartered in Winsted, Connecticut, appears to be a legitimate operation -- we have little doubt that your pre-purchased copy of Halo 3 would be shipped soon after the official release. As an added bonus, Lion Games accepts trade-ins (you pay for shipping). So as an alternative, you could pile up enough games and send 'em in to cancel out the Halo 3 charge -- heck, you could even trade in enough product to get Lion Games to cut you the check.
Let us know how the exchange goes.