When historical Snow White variants are discussed, you generally hear that Basile's The Young Slave is the oldest known literary variant. Kate Forsyth recently had a Snow White feature on her blog; if you haven't read her spotlight blogs, I highly recommend them. She discusses a comprehensive history of each tale as well as the most common ways of interpreting the themes. They're each very well researched-you can also read her spotlights on Little Mermaid and Sleeping Beauty.
She included this tale which I don't think I've heard of before:
"The oldest seems to be the medieval Norse saga written by the 12th century poet Snorri Sturluson, which sets the tale in the time of Harald Fairhair in the 9th century.
The story is called ‘Snow Beauty’, and tells the story of how, one snowy winter’s day, Harald Fairhair fell in love with the most beautiful woman in the world and married her. When Snow-Beauty died, however, her body did not rot and her cheeks were as rosy as they had ever been. The king sat beside her, thinking she would soon come back to life. He sat so for three years, neglecting all his kingly duties, until his wise councillor bade him lift up the dead queen so they could change the bedclothes below her. As soon as she was lifted up, a rank smell of rotting rose with her, the body turned blue, and worms and adders and frogs and toads crawled out. So she was burned, and the king returned to his wits."
So it's not exactly a Snow White tale, but with important similarities. In fact, in Once Upon a Time: On the Nature of Fairy Tales, Max Luthi discusses how tales of unnaturally long sleep are handled differently in fairy tales verses saint's tales or legends. In both there is a supernaturally long sleep, but in the fairy tale world, once awakened, the character goes right on living as if she had only gotten up from a nap; she hasn't aged or suffered any physical consequences.
But in other types of tales, someone might fall asleep in the woods, wake up and return to their town, to find to their surprise that it's all full of unfamiliar faces. He asks around for friends and acquaintances and finally learns that someone of his very name had lived in the town years ago, disappeared, and was assumed dead. At that time he realizes he was asleep for an unnaturally long time, and upon the realization, time catches up to him, and he ages and dies. Snow Beauty fits this second kind of tale better-it's more about the inevitability of the passage of time (or even the grieving process, for the King), a more creepy story than even the popular dark fairy tale. But of course, being in a different genre doesn't mean it's not a related or influencing tale; many fairy tales have motifs in common with myths and other ancient stories.