How to Diagnose Dishwasher Problems | eHow.com:A child's cup top had jammed the water-level float full open. The dishwasher wouldn't fill because its sensor said it was already full.
...Dishwasher won't fill Inlet valve or float switch is malfunctioning
Clean or replace valve or float switch...
Easiest $60 a repair guy ever makes. They have to make a living, so I don't feel too bad that they didn't suggest we check the float when we phoned.
The real lesson though is always google first. I always remember that when my computer hiccups. I don't always remember it when the dishwasher is dry.