From www.smallvilleph.com:
"This power was first demonstrated briefly in "Metamorphosis" (Season 1, Episode 2), where Clark was floating on top of his bed and later in "Rosetta" (Season 2, Episode 17), where Clark actually floated, rather than fly, from his bedroom to a highway. Clark has mentioned that he told Lex, who found him and drove him, that he sleepwalked, although he "has no idea how much walking is involved". If this is true, then it should not come as a surprise that Kal-El is able to fly in "Crusade" (Season 4, Episode 1)."