http://xbox360.ign.com/articles/106/1061465p1.html
That's right, a blog on Sega's website confirms that they are making a new sonic game, and it WILL be just like the original.
What do I mean by that? That means 2-D only gameplay, Sonic as the only playable character and a lack of appearences from other characters such as Big the Cat. Holy crap, maybe sega did learn!
What platform will it be on though? It'd be hard to make a full fledged 360/PS3 title on 2-D titles, and they have never existed in the platforms save for the recent Street Fighter 4 and downloadable games. So, first estimate goes to the Wii, which actually has had success on full 2-D games on an actual disk. It would also make sense to make it a Nintendo platform since Sonic sees the most success on them.
Of course, it could also be a downloadable game, which makes it possible and likely to be on the 360 and PS3. Let's hope it's good.