The chief among the list of updates has to be the inclusion of two new playable characters form Sonic's universe: Knuckles the echidna and Shadow the hedgehog.
Other new additions include a new course (Sun Fair from Samba de Amigo) and the ability to purchase Sega miles as an in-app purchase. 500,000 miles go for $.99 a pop, followed by 1,500,000 for $1.99, and 50,000,000 for $9.99 (all USD).
For the other smaller additions see the official description from the App Store:
The Knuckles & Shadow UpdateQuite an update, no?
Two New Characters – Knuckles and Shadow!
One New Track!
iCloud Progress Saving!
Character Faces on the Minimap!
Game Center Profile Pics on the Minimap in Multiplayer!
And lots of other improvements and bug fixes!