Indeed I had NSS in mind, moreover it would be great if the shield could be extended to hold 2 hardware enabled RS232's . room enough...
That would enable to place the Arduino between two serial devices to monitor communication, optional to modify it. It would be neat if the CTS/RTS pins
Another application is the multiplexing of the serial signal especially TX, e.g. one Arduino sending to 2 PC's .
INstead of 2 RS232's I can imagine to have all 9 pins supported, including lines as Ring Indicator etc. prefeably with the option to do some loopback, like connecting the RTS and the CTS line so if a device sets the RTS it gets immediately/automatically a CTS. Sort of breakout box functionality.
1 DCD - Data Carrier Detect
2 RX - Receive
3 TX - Transmit
4 DTR - Data Terminal ready
5 GND - Ground
6 DSR - Data Set Ready
7 RTS - Request To Send
8 CTS - Clear to Send
9 RI - Ring indicator ==> to arduino interrupt pin
e.g. see - http://www.lammertbies.nl/comm/cable/RS-232.html
just thinking out loud ...