Hiroshige Goto (via IGN), Sony is looking into using an upgraded version of the Cell processor for the PlayStation 4.
Goto speculates that Sony's upgraded processor would feature between 10 and 20 cores (current Cell uses 8) and boast twice the power. Keeping with the Cell -- as opposed to developing a new chipset -- would not only be cost efficient for the company, but it would also likely prove beneficial to manufacturing costs, software development (it'd be a familiar architecture) and backwards compatibility. Remember: although this is from a reputable source, the news is unconfirmed and, given the PS4 is likely many years away, subject to change even if true.
Fun fact: There are approximately 1900 days (over 5 years) between the PS2's March 2000 launch and Sony's E3 2005 unveiling of the PS3.