Screen as a serial terminal emulator in MacOS X

screen /dev/tty.usbserial-XXXXXXXXX 9600

More comfortable: Apple script

tell application "Terminal"
do script with command "screen /dev/tty.KeySerial1"
set number of rows of window 1 to 100
set number of columns of window 1 to 80
set background color of window 1 to "black"
set normal text color of window 1 to "green"
set custom title of window 1 to "SerialOut"
end tell

Control-A followed by Control-\ to exit your screen