Frankie has posted at Bungie.net that Best Buy may be your best friend come Halo 3's launch day:

During the launch period of Halo 3, the 25th, 26th and 27th of September, Best Buy will be sponsoring the Xbox Live Gold service and making it free for everyone. Think of it as Halo Free Gold Days. That means that even if you only have Xbox Live Silver, or you picked up a 360 just for Halo 3, that you’ll be able to try almost all of the features – online co-op, Multiplayer, Forge editing, viewing networked Saved Films - however, you will not be able to use our File Share system unless you subscribe to the paid Xbox Live Gold service.

Interesting restriction. Looks like nobody wants a lack of a Gold subscription to get in the way of Halo 3's online multiplayer numbers.