I think that Boo is one of the two main mockingbirds in the novel. Because of problems in his family, possibly his mental state - and who knows what else Boo thinks about the society because we're not privy to his thoughts - he, eventually, voluntarily avoids society. As the Character Analysis Link says, Boo, like Tom performs acts of kindness without expecting reward. Boo leaves things for the children in the tree, covers Scout with a blanket, and eventually rescues Jem. Tom helps Mayella because he feels sorry for her. Boo and Tom are the mockingbirds: Boo, somewhat voluntarily, becomes a recluse because society won't accept him. Tom is involuntarily banned from society (to prison) because society won't accept him or his honesty. In each case, they are innocent (they don't do "anything but sing," and therefore, why should society bother them) but are outcast. The attention paid to Boo does many things in the novel and one of them is to set up the image of how society can make someone an outcast or outright remove them from society based on fear, ignorance and misunderstanding.
Aside from the Finches--Scout, Jem and Atticus--Boo Radley is one of the pivotal characters (along with Tom Robinson) of To Kill a Mockingbird. Boo's character is introduced through the tales of Jem and Scout, and when Dill enters the picture, the threesome get down to the serious business of luring the most secretive of the Radleys out into the open. Boo is used in many ways throughout the novel: He serves first as a distraction for the kids during their summers, but when they discover that Boo is not the gruesome, squirrel-eating ghoul that his legend suggests, they restrict their activities concerning him.
Boo serves as a sort of transition between the first two parts of the book; the focus on Boo shifts to Tom Robinson in the second part. The author, Harper Lee, spends so much of the first chapter on Boo in part because he is so mysterious and interesting. The fact that he does not actually appear until the final chapters helps to maintain this interest until the end.
I think that so much time is spent on Boo Radley in the first chapter because he is one of the main focuses of the book, especially in the part before the Tom Robinson trial starts to become a big deal.
Boo is a symbol of the inner fears and insecurities that the children have. Much of the book has to do with how they work out those problems.
Boo is also a symbol of weakness and being outcast. Part of the book's message is that the weak should not be picked on and cast out. By emphasizing Boo, the author starts in on this theme right away.
I believe that Boo Radley is mentioned so often in the opening of the novel because he is an essential factor in the growth of Scout/Jem's maturity throughout the novel. As the feared scapegoat of the indolent society, Boo's inferiority (as labeled by society) relates directly to the harsh, supercilious views Maycomb imposes upon African-Americans who have done nothing.
Atticus tells the children never to kill a mockingbird because they have done nothing but provide lovely melodies for people to listen to - because they are innocent. Boo Radley and the African-American community both symbolize mockingbirds in that they have done no wrong deed yet are blamed for all the failures of society. The children, believing the unlikely rumors exchanged throughout Maycomb, are at first frightened by this mysterious figure. Their childish curiosity leads them to accept a number of dares, each involving Boo Radley (touching the Radley Place, sending him a special message through the usage of a fishing pole, etc.). However, as the novel progresses and the trial of Tom Robinson arises, the children are able to relate the similar injustices both Tom and Boo are forced to face because they are different from the typical white mold of society. THe society prefers to remain loyal to their southern traditions and reject those who act as iconoclasts to their quintessant society.