It's not a "viable" model for the /developers/ involved in the Humble Indie Bundle. Not in the sense of pricing for /new/ releases, at least.

However, each of these games have been out for sufficiently long that having people name their price now is mainly extra frosting on top of an already well-decorated cake. It's a tiny bit of extra market penetration /after/ the game's already made an impact - along with some goodwill towards deserving charities.