Snow elves came from PC gamers reading all the data from patch 1.5, which mentions a snow elf prince and his spear. Everyone assumed they'd be in Dawnguard.
Shortly after Dawnguard was announced, Bethesda copyrighted the name 'Heathfire', so i'm guessing that's DLC#2.
Cannot wait for Dawnguard. I'm going to have to play it twice though, once as a vampire and once as a vampire hunter.