First guy that came to mind of "bust" to "sleeper" is Chad Billingsley. He is going crazy low (got him in the 21st round of a 1st yr 10 team dynasty league), but rightfully so, as many projected him to be a top 25 SP the few years and have been burned hard (including myself). However, he is still just 27 years old. Had a 4+ era last year, but his FIP was in the 3.80s. His walk rate increased last year, and his strike rate decreased; two stats going the opposite way. So maybe it is just blind faith, but when you are drafting him that late (was my #7 SP with 30 man rosters), it is a great low-risk/high-reward scenario.
Guy that went from sleeper to potential bust is Brett Lawrie, no question. He is being drafted too high to the point where a 20+/20+ season is expected or your are not profiting, and in reality he is still growing as a young hitter and his HR/FB was unsustainable. I could see him going 15/15 this year, frustrating his owners, and becoming a sleeper next year.