Actually, it's all about anti-counterfeiting measures.
Yesterday the new $100 bill was officially unveiled by the United States Department of the Treasury, the Govenors of the Federal Reserve and the United States Secret Service. Where would such an event take place? Where else but the Department of the Treasury's Cash Room. The high-tech bill is loaded with new security measures, including a 3-D security ribbon that changes images as you move the bill around (special glasses not required). Check out the liberty bell-- it changes colors from copper to green and actually looks like it disappears when you tilt the bill. For an inside look at all the new features, check out the animated, interactive tutorial
And oh, what about the other 6.5 billion $100 notes already in circulation? According to Secretary of the Treasury Tim Geithner, all U.S. currency remains legal tender, regardless of when it was issued. This is the final bill in the family of redesigned notes, ending a project that began with the new $20 bill introduced in 2003.