I know I am joining the discussion relatively late, but I will respectfully disagree with Stefano. My players tend to like the cameos. It adds flavor and reinforces the idea that this is a Forgotten Realms game, the same forgotten realms these famous characters are from.
Instead of running into Obama getting a cheeseburger, it would be like being a nightclub magician/kiddie-party-magician and running into Penn and Teller getting lunch, which is very possible if you live in Vegas.
The key is to make sure they are cameos. Seeing Drizzt giving a lesson to some of the Knights of Silverymoon is nice flavor. Having him come to the character's rescue when they get overwhelmed in the region of Silverymoon might make the characters feel less heroic and might feel a bit deus ex machina, but it could still be an interesting scene if handled right. Having him actually travel with the group on the otherhand will horribly distort the game unless the characters are very high level.
Similarly, Alustriel isn't going to go meeting with every adventurer out there, but she no longer adventures personally, and in a time of crisis it is quite believable that one of her personal advisors might hire low-level character's to handle some of the side work while the famous heroes handle the maint thrust. ("Althuzimel the ancient dragon has arisen and gathered an army of followers! Drizzt and his group are going after the dragon, the Knights, under Alustriel's personal supervsion, will be handling the armies main thrust. But we need you to handle the east flank where we expect twenty orcs under Althuzimel's command to attack the outlying farms!") Done right, this is quite believable, injects a Forgotten Realms flavor intot he campaign, and makes the characters feel like part of something bigger, but without letting the famous personalities overwhelm the game or letting the characters part be unimportant. (Alastriel herself might never notice the characters that protected the farms to the east, but her advisor will pay for their work, and the actual farmers will be eternally grateful and they still participated in events that involved Drizzt and Alustriel.)