This is only coming to Windows for the time being, but Dodrill has not entirely ruled out Mac or Linux ports. He's also confirmed that Games for Windows Live won't be used; instead, Dust will rely on Steamworks for achievements and cloud saving, and it'll be "constantly supported."
Along those lines, there are plans to put out a title update for the original Xbox Live Arcade version which "includes all updates from the PC version." I hope this game continues to do very well across any and all platforms it hits.
Spelunky updates: PS TV support, replay fast-forwarding, Daily Challenge ghosts
7:00 PM on 11.10.2014