In a recent interview, Sega's Yuji Naka of Sonic Team fame went on record stating that the company plans to release two more Sonic games this year, both for Nintendo consoles.
The recently announced DS game will most likely be a faithful platform outing, though no details have been confirmed details at this stage. Even less is known of the next GameCube instalment, but a direct sequel in the Sonic Adventure series is expected by most.
Stay tuned for updates.