They could be round, for easy handling. And milled for evidence of having been shaved. They could even be made of precious metals for high-value coins, and of base and inexpensive metals for low-value coins.
This would solve the telephone privacy issue.
However, they did have other problems. We once had a cypherpunks meeting
at Soda Hall in Berkeley, and unlike the usual problems finding parking,
I was pleased to find that a bunch of spaces on the street that used to have
parking meters had the working parts removed and replaced with flowerpots.
While the flowerpots were a nice Berkeleyish touch, the basic cause wasn't
a desire to have unrestricted parking, it was a discovery by teenagers that
there were pots of money sitting around waiting for people with metal pipes to collect them.
Pay phones also have this problem :-) They also have the problem that it costs money to send people around to collect the coins, as opposed to collecting data over wires you've already got, and then there's the problem that there are people driving around in trucks full of money... and of course the problem of deciding whether round pieces of metal have the right politicians' pictures on them without cryptographic help.
On the other hand, by switching the money part to coinage, it would free up the data connections for the surveillance cameras.
