If bitcoin booms it's never going to completely replace dollars. I can see it being more like;
"That'll be $3"
"Cool, bitcoin ok?"
"Sure"
*puts $3 into bitpay or some form of payment processor that converts it to bitcoin at whatever the going rate is*
"That'll be 0.006btc then"
"oh wait, bitcoin just plummeted, that's .01 btc. thanks.
"