The 2nd gen lasted 6 model years, so I wouldn't expect to see a 4G Prius until 2015 at the earliest. They usually do some sort of mid-cycle refresh somewhere in there. With the 2nd Gen Prius they did it in 2006.
MFD upgrade, different headlamp and taillamp housings, back-up camera, chrome upper grill and lower front bumper trim, also changed how things are laid out on combination meter and interior design finish.
Only rumors Likely very minor change in 2012my in 2011, when they release the plug in hybrid. More major change in 2015my in 2014. Its hard to tell how major. I would expect different battery packs and option packages. I would not expect any major change to the body.
Typical Toyota lifespan is 5 years. So far, both Prius generations have lasted 6 model years (Dec 1997 - Sept 2003, Sept 2003 - May 2009). A midlife update is usually in between (3rd or 4th year typically).
