The BBFC classification site lists a "Sonic Classic Collection" coming to DS soon. Normally, with these ratings, we'd be left to speculate as to the identity of the games on the collection -- however, the BBFC helpfully provided a list of games on the cart!
According to the rating, it'll feature Sonic 1-3, plus Sonic and Knuckles -- with the ability to virtually "Lock-on" and play as Knuckles in the older games. Also included in the shockingly specific list of in-game content: Sonic Chronicles and "The History of Sonic" video content and the art gallery.
We can only hope it turns out better than Sonic the Hedgehog Genesis on GBA.