I am using a PL-2303 USB-to-serial adapter when connecting to serial interfaces.
After having installed Snow Leopard and reinstalled the Prolific PL-2303 driver (32-bit version from prolific), I found myself prevented from lauching cu to connect to the serial port:
elasticsheep:~ mathieu$ cu -s 9600 -l /dev/cu.usbserial cu: creat during lock (/var/spool/uucp/TMP00000000c8 in /dev as uid 501): Permission denied cu: /dev/cu.usbserial: Line in use
It appears that permissions have changed from Leopard and I am now not allowed to execute cu as a simple user (as mentionned here).
The solution is to use sudo to launch cu:
elasticsheep:~ mathieu$ sudo cu -s 9600 -l /dev/cu.usbserial Password: Connected.