Starting July 26th in Washington state, stores with three or more employees working at the same time must allow customers access to an employee restroom so long as it doesn't pose a security threat. Businesses also have to provide bathroom access to anyone with an inflammatory bowel disease who can present a card or signed statement from a doctor saying they've got a condition.

In practice, we bet a lot of stores will play the "security threat" card to any request and things will continue as they've always been. Maybe if you printed out these two stories to show an employee when they ask for your papers—

"Jo-Ann Fabrics Refuses To Let Customer Use Bathroom, Even As She Suffers Diarrhea Right In Front Of Them"
"Rocky Mountain Chocolate Factory Refuses Bathroom Access to 5-Year-Old, Who Then Has Diarrhea In Front Of Them"

—you'll be taken more seriously. Especially if you start to drop trou as they consider their options.

"Gregoire signs bathroom access bill" [Seattle Times] (Thanks to Casey!)