"In August 2007 Sheldon was diagnosed with primary progressive multiple sclerosis. After losing his ability to balance an upright bicycle to the disease, he was able to continue pedaling by using a recumbent tricycle. Sheldon died on February 3, 2008 after a heart attack."
More info from other blogs:
The Daily Randonneur
Tin Donkey Travels
Sheldon, I never met you in person and I regret it. You have been a great influence on the many worlds of cycling, both old and new. You will be truly missed.