Just looking at this decade, there's been a few teams make UNRANKED runs to the tops of the polls:
2014 - TCU, unranked until early October, finishes 3
2013 - Auburn and Michigan State, both unranked until mid-October, finish 2 and 3
2012 - Texas A&M, unranked until early October, finishes 5
2011 - Michigan, unranked until mid-September, finishes 12
2010 - Nevada, unranked until late September, finishes 11
2009 - Pitt & Wisconsin, unranked until late October and early November, finish 15 and 16
2008 - TCU, unranked until mid-October, finishes 7
2007 - Missouri and Kansas, unranked until Sept and Oct, finish 4 and 7 (BYU went unranked until Thanksgiving and finished 14)
2006 - Boise State and Wisconsin, unranked until Sept and Oct, finish 5 and 7
2005 - Penn State, West Virginia and Alabama, unranked until Oct, Oct and Sept; all finish in top-10 (3, 5 and 8)
2004 - Virginia Tech, unranked until mid-October, finishes 10
NOTE: Georgia Tech, 1990, last team to win national title (UPI poll, w/ Colorado the AP), was unranked until the poll on October 2. Debuted at 23, went to 18, then 11, fell to 16, climbed to 7, then 4, then 3 then 2 and won its bowl game to share the title.