Of course any game that is going to sell between 2 and 3 million copies in a matter of hours is overhyped. No game, not even Halo deserves that.
However, there are far too many games out there that don't deserve their breakneck sales - Nintendogs, Wii Sports, Pokemon, Madden, Dragon Quest (atleast in Japan), Final Fantasy (the most recent ones), and so on.
However, why care? If you don't feel Halo3, or any game I mentioned isn't going to live upto the hype, then feel free to not buy it. If you think Halo3 isn't a system seller, then don't buy an Xbox 360 for Halo 3. It's that simple.