Like CRT to LCD, standard def television to HDTV, DVD to BluRay, the "king of the hill" electronics change, sometimes slowly, sometimes rather fast depending on many factors. Consumer adoption sometimes drives the timeline. In electronics it can often be parts availability. If the chip makers discontinue designs, the electrical engineers must change their designs or maybe discontinue the product.
5 volts has been the electronics standard for powering chips for decades. Since the 7400 series discrete logic chips, you had to have 5 volts to power devices and talk between chips.
But 3.3 volts (often abbreviated 3V3 or 3v3) has been growing strong in recent years. This has accelerated with mobile electronics growth, lower voltage reduces power and heat loss. Many popular systems are now 3 .3 volt including popular XBee radios and many flavors of microcontrollers.
But 5 volt chips do not talk to 3V3 chips well. You can use a number of engineering tricks such as voltage dividers or FETs or go elegant with a level converter chip in between.
Many of the new systems are coming out in 3V3 power including the Raspberry Pi (internally, 5V from the tap), and the new Arduino Due (the new spec sheet has a warning to ensure folks know this fact).
It certainly looks like 5 volt designs will continue to wane. It may be awhile before it joins VHS tapes and CRT displays - you should start to plan your designs for the 3.3 volt future.