The Ubisoft product page has a fairly detailed list of features for the game. Players can now record their "freestyle" creations and make them into playable levels for other players. A multi-track song editor is now included. In addition, there are now multiple selectable guitars, including Fender and Gibson models, and both acoustic and electric guitars. Jam Sessions 2 features songs from artists including REM, Heart, The Ramones, The Clash, Doobie Brothers, Plain White-T's, The Black Crowes, and The Pixies. Some kind of DSi-based "features to enhance sound effects while playing" are also included.
Ubisoft lists a September 29 release date for Jam Sessions 2. We're contacting Ubisoft to try to learn more about this currently unannounced title.