The rarity tin will be available at available at Target stores soon, and will include Promo #5 "Rarity, Dragon Charmer". No information about this promo is available yet and is definitely one to watch. If this tin is the only source of the Promo, expect it to be worth a considerable amount.
The Twilight Sparkle tin will be available at Hot Topic stores, and will include a copy of Twilight Sparkle Research Student. This promo has already been released and is relatively cheap on eBay at the moment, so this tin probably won't be in great demand.
*EDIT* Enterplay just spoiled this Princess Luna promo card from Canterlot Nights. I'd be willing to bet that this is the promo included in the Luna collectors tin.
I'll be keeping my eyes and ears out for release dates on these collector's tins and will post a new article as soon as more information is available.