Wild Arms 5 is officially under development, according to Sony's Wild Arms website. New flash on the site say that Wild Arms: The Vth Vanguard is coming.

No specific release date is mentioned, but the flash opens with this phrase: March Winds and April Showers Bring Forth May Flowers. Perhaps May is when it is coming, or maybe that's just when it will be unveiled.

There is a bit of story unveiled in some text at the end of the flash movie. Here it is:

The planet named Fargaia slowly but surely loses its green and changes to the wasteland Severe environment and the cruel monsters threatens life, people still hang on the planet., a wild world, there are adventurers who ask for a thrill or a dream. People call them "Migratory Bird."

Wild Arms [Official Site]