The "Noble Map Pack" is set to be available on the Xbox Live Marketplace on November 30, at an asking price of 800MS Points ($10). The content will also add an additional 250 achievement points to Halo: Reach, which explains a lot.
Noble's three new environments are "Anchor 9," a 2-8 player map that takes place on a UNSC space station dry-dock; "Tempest," an 8-16 player abandoned shoreline facility area with Forerunner technology lining its two symmetrical bases; and "Breakpoint," a series of modular archaeological labs, atop an icy precipice for 8-16 players.
Bungie has also kicked off a map creation contest for its community, dubbed the "Forgetacular Map Design Contest." Details for the competition can be found on a dedicated blog post featured at Bungie.net.
[Ed. Note: The "Noble Map Pack" will launch on November 30, not on November 20 as originally listed]