I understand everyone runs out of cash from time to time, so I would understand this happening every once in a while. But it seems to happen all the time: a whole line of people using plastic for $3 lattes. Turns out this hasn't been my imagination: Card Transactions Hold Majority in U.S.(no direct link to article)
For the first time, Americans used cards -- credit, debit and others -- to buy retail goods and services more often than they used cash or check in 2003.The article does point out that some fast-food outlets, like McDonalds, have waived the signature requirement, speeding up the transaction, but it is still much slower than cash. The coming use of cellphones for an electronic purchase, however, does seem be the right model since in this case the transaction would be more immediate and take the same time as a cash purchase.
I'm one of those people who usually has a ready supply of cash on me for my day-to-day purchases, but I am definitely becoming a minority.