No, that's not a typo. As of this writing, Super Smash Bros. Brawl is available for pre-order at Wal-Mart.com for under $20 (shipping raises it by a couple bucks). Limited quantities? Bait and switch? Colossal corporate screw-up? Either way, you probably should act fast if you're interested.
(In fact, the site has several games available for pre-order well under market value, such as Haze and The Club. That lowers the likelihood that somebody messed up.)
Update: They fixed it! It's now $49.82.