I see from the trailers for the new Hobbit film that Frodo Baggins is in it.
It has been a while since I read the book but I don't remember Frodo being in it.
Was his addition only for the movie adaptation or was he in the story all along?
It looks like Frodo is being shoe-horned into The Hobbit to keep audiences happy (general audiences, mind you, not LotR fans who are not pleased with this news).
According to a report from AICN:
What’s Frodo doing in The Hobbit? I don’t want to spoil too much, but I can say that Frodo is part of the connecting tissue between The Hobbit and Fellowship of the Ring. In fact, the next shot was an over the shoulder on Elijah Wood hammering a sign up on Bag End’s front gate: “No Admittance Except On Party Business.” You guys should have an idea where that puts this moment in the timeline
Extrapolating from this tidbit, I believe it is safe to say that 'Elderly Bilbo' (Ian Holm has already filmed his scenes in London) will be recounting his tale to Frodo before the party that we see in Fellowship of the Ring. The events of The Hobbit took place 60 years before the Lord of the Rings trilogy (and 28 years before Frodo was born) - so this explanation seems to make the most sense.
In answer to your original question - he was only added for the movie adaptation.
BTW - the same goes for Legolas...