Status changed from 'Started' to 'Completed' - Sorry forgot to update this thread. It was actually completed a few weeks back. It's not fully integrated on achievement detail pages, but is visible on member achievements and game achievements pages.
Rare achievements (as deemed by Xbox.com) have a diamond icon next to the rarity percentage. Keep in mind we are still displaying the rarity percentages as determined by the members of the site, not the Xbox.com rarity percentage. This may change in the future but for now that's what we're doing.
Also, since this feature is still relatively new it is considered to be in beta status for Xbox Live, and no badges are planned in association with this update. Again this may change in the future as this feature develops further.
Join the official Gamertag Nation club on Xbox Live here.