According to USA Today [usatoday.com]:
Stephen Hillenburg, the visionary creator of "SpongeBob SquarePants" died Monday at the age of 57. The cause of death was ALS.
SpongeBob is Nickelodeon's biggest all-time hit, and has aired for nearly two decades...
Hillenburg first parlayed his fascination with ocean life into a career as a biology teacher...
In 1987 he began his career in animation, and from 1993 to 1996 got his start at Nickelodeon on Rocko’s Modern Life.
A different story about Spongebob rumors and guest starts is reported here [usatoday.com].
News of his death also reported at ABC [go.com], Washington Post [washingtonpost.com], and NBC [nbcwashington.com].