This isn’t really a HUGE surprise since this was just bound to happen. No word yet on a release date but it could be as soon as October. You will also be awarded with ten more achievements, bumping the 1000 possible gamerscore to 1250.
“We plan to extend the online experience of ‘Halo: Reach’ with a robust lineup of Add-on content for the most popular game on Xbox LIVE today. While we don’t have any specifics to share at this time, we look forward to revealing additional details on new Add-on content for ‘Halo: Reach’ in the near future.” – Microsoft