Sonic Generations is laying the nostalgia on pretty thick. Between old-school chubby Sonic and clear reference Green Hill Zone, Sega is making sure this hits the part of your brain that pines for the 16-bit era. And since the game is already steeped in retro-goodness anyway, they must have figured "what the heck?" and thrown in the original Sonic the Hedgehog for fun.
Destructoid reports that Sonic Team's Takashi Iizuka revealed the bonus at a panel during Paris Games Week. This way you whippersnappers who don't actually remember the original can see what gaming was like back in our day. For example, you might not realize that we had to walk uphill through the snow, both ways.
Shacknews has contacted Sega of America to confirm the bonus inclusion, and will update as more information becomes available.