The $100 is a favorite of counterfeiters, for obvious reasons, and the government's rolling out new bills to try and make life harder for them. It seems like the old antibiotics-versus-bacteria thing, where the pathogens/counterfeiters are always catching up to the drugs/government.
Here's a brief description from the feds of some of the (literal) bells and (metaphorical) whistles on the new bills:
The blue 3-D Security Ribbon on the front of the new $100 note contains images of bells and 100s that move and change from one to the other as you tilt the note. The Bell in the Inkwell on the front of the note is another new security feature. The bell changes color from copper to green when the note is tilted, an effect that makes it seem to appear and disappear within the copper inkwell.The new hundreds will enter circulation in February of next year.
For additional information go to NPR's Planet Money.