If you're lucky, your local Wal-Mart might already have Guitar Hero 2 on sale ($79.99 MSRP). It would seem that certain outlets have already received their copy and put them out nearly two weeks before the actual street date, Novemeber 7th.
Of course, in any situation like this, your mileage may vary. Some stores have yet to receive stock, while others are keeping theirs behind closed doors claiming that they can't sell it for another couple weeks.
Best Buy pulled a similar stunt with the first Guitar Hero, putting copies out on shelves only a week before the game's release. There are no reports of the chain repeating its mistake.