U.S. paper currency has been going through a redesign process for a while now... when the new $20 bill first came out, it took some getting used to, even for a world-traveller like me! Anyway, here's the official line from the U.S. Treasury:
The Redesigned Currency: Safer, Smarter, More Secure. In keeping with the strategy of maintaining the security of our currency by enhancing the designs every 7 – 10 years, a new series of U.S. currency is being issued, beginning with the $20 note which entered circulation on October 9, 2003, followed by the $50 note which was issued on September 28, 2004. The most recent denomination to be introduced in the series is the $10 note, which was issued on March 2, 2006. The redesigned $5 note is scheduled to be issued on March 13, 2008. The $100 note is also slated to be redesigned, but a timetable for its introduction is not yet set.