Science has picked the rapidly expanding field of cell reprogramming as its scientific breakthrough of 2008. 

First described in Shinya Yamanaka's landmark 2006 study, reprogramming turns back the cellular clock - transforming specialised adult cells into embryonic stem-like cells, called induced pluripotent stem (iPS) cells.  Several developments in 2008, including the creation of disease-specific iPS cells, are detailed in a news article by Gretchen Vogel surveying the year in reprogramming (free access, registration required). 

The Science website also hosts a video featuring Vogel, George Daley and Shinya Yamanaka, reviewing some of the work that led studies in reprogramming cells to be tagged the top scientific story for 2008.