It will be available first on Steam and GOG.com, and later on the Humble Store. The studio previously announced that ports of the XBLA version would be released this summer on PlayStation 3 and PlayStation Vita via PlayStation Network. Mossmouth's Derek Yu told Polygon during GDC that the PS3 and PS Vita ports would be handled by a different studio; we've reached out to the company to ask if the same is true for the upcoming PC version.
Spelunky was originally released on PC in September 2009; that version, which Mossmouth now refers to as Spelunky Classic, will remain free.
Update: Yu told Polygon that port specialist Blit Game Studios is developing the PS3 and PS Vita ports of Spelunky, while Mossmouth is handling the PC version in-house.