The Heroic map pack gets released this month (December 11th). Some
information on it.
I think it becomes a free download in Spring of 2008, so you can either spend the 800 credits or wait till next year to get your hands on the new content. Going on how dedicated the Halo community is I reckon there will be plenty of early adopters making this a worthwhile purchase. Looking forward to seeing the return of the soccer ball. haha. Apparently there are a few other easter eggs included in the new maps.