Sega promised to post a teaser trailer for Sonic the Hedgehog 4 Episode 2 today. That they did. However, there is no gameplay. There are a few teases of things to come however. Metal Sonic has been confirmed as an antagonist (hinted via connections to Sonic CD). Tails will also be involved in some ways, likely as a partner character similar to Sonic 2 (perhaps in playable form, we'll see).
Prior to this teaser, it had been rumored that Episode 2 would heavily improve the physics engine to be more like those for Classic Sonic in Generations. Also with the rumor came word of "less glossy" visuals. In an interview with Sega's Ken Balough (nice mustache btw), he confirmed that the game would indeed receive improved physics as well as visuals. We shall soon see what this means specifically. Many fans were turned off by the physics in Episode 1 (as has been a common complaint with Sonic games for most of the last decade). You can check the source below for the full interview.
Sonic 4 Episode 2 has a tentative release of simply "2012". In the logos, it has been confirmed that it will be coming via download to Xbox Live, Playstation Network, as well as the Android Market, iOS App Store, and even the Windows Phone. Unfortunately the game will not be hitting the Nintendo Wii. No logo for it was found at the end and Sega has confirmed it will not be coming to the Wii (for "reasons people have probably figure out"). No word on the 3DS or Wii U however. Wii U is releasing next year and so will Episode 2, so perhaps there's hope for that platform, and maybe even 3DS. Stay tuned for future news.