Sega has the PlayStation 2 version of Sonic Mega Collection Plus on display, though it was only 20% complete. The E3 build allowed you to play Sonic the Hedgehog, Sonic 2, Sonic 3, and Sonic & Knuckles. They all played exactly as you remember them.
A complete list of titles is unknown, though all of the games from the GameCube version are to be expected. As for what gives Sonic Mega Collection Plus the plus, the Sega rep we talked with was unable to comment on any specifics. Current speculation is that the Game Gear titles in the GameCube version of Sonic Adventure DX will be included, though most hope that Sonic CD will be added.