Shouting "I'm going on an adventure!" in the Shire is like shouting "I'm going to an orgy!" in a Catholic Church.
The Book: Bilbo wakes up alone, and after realizing the dwarves are all gone, feels a pang of disappointment that he didn't tag along. He cleans up the dishes they had left behind from breakfast, makes a breakfast of his own, and was sitting down to enjoy a second breakfast when Gandalf barges in the door. Gandalf points out the message left on the mantelpiece by Thorin, instructing Bilbo to meet them at the Green Dragon Inn at 11; it's 10:50, says Gandalf, and Bilbo would have to run. Bilbo tries to argue, but Gandalf yells at him to hurry up, and Bilbo rushes out the door without a second thought.
What difference does it make? This is further changing the characterization of Bilbo Baggins. In the movie, he changes his mind only once - but it's his choice. The book has him flip-flopping a few times, and in the end, the reader isn't even certain that Bilbo made the decision himself. It appears that he was simply bullied into running out the door in order to make the appointment. Giving him time to prepare, though, was an odd choice. Bilbo's rushing out the door without so much as a pocket-handkerchief is part of his overall character arc; he had to leave behind all the comforts and safeties of home, and start to learn to live entirely on his wits and mettle. A filled backpack somewhat takes away from all this.
My Opinion: As much as I appreciate Bilbo being pressured into going on a journey that he (initially) didn't want to, simply because he was too polite, I like the movie's version better. Having Gandalf literally show up and scream at Bilbo until he immediately left takes away from the decision to go. It should be Bilbo's decision, and the movie made it clear that it was entirely his decision. It's a rather sizable change, in my opinion, and one that makes me a bit leery (especially the part where Bilbo rushes out), but it's nonetheless a change for the better.