http://groupees.com/bm11Guns of Icarus Online will be unlocked as the last bonus when 25,000 bundles have sold. So far, the bundle has sold some 21,000 in four days, and it ends in 10 days. So it's very likely that the bonus will be reached.
Buy the bundle for $1.00 to get all of the games and their Steam key (all but one game are on Steam), including GoI when the bonus is reached. If you've been looking to get your friends into GoI, this is probably the cheapest it's going to be for a while.
Again, GoI is not unlocked at the moment, but it is extremely likely that it will be in the next couple of days.