The game that's now coming to Linux over Valve's Steam distribution client is Air Conflicts: Pacific Carrier. This is a 2012 flight simulation game that's set back in World War II aboard a variety of aircraft carriers. This is one of the sequels to the 2006 game Air Conflicts. This flight simulator game features both single-player and multi-player modes.
It's been announced at SteamCommunity.com of the Linux (and OS X) port. The Linux and Mac ports are already complete and just waiting on approval from the publisher before they're launched. The developer of the game says he hopes it will happen really soon.
While Air Conflicts isn't the most exciting or advanced flight simulator I've come across, it is a nice alternative to all the first person shooter games on Linux. The game will be available through Stream for a price of $30 USD.