Super late to this conversation I know, but I have to add that you should NOT hook a regular RS232 serial device up to your iPhone's serial port directly! The iDevice serial port is a TTL port, and the voltages in regular serial will probably destroy your device if you do.
You should use something like a MAX3232 chip to convert between the two, or buy a prebuilt cable with one installed in it. Use something like Minicom, and as was mentioned, change the device to /dev/tty.iap and it'll work fine.
Google for 'iphone serial port' and you'll find plenty of results. I console into Cisco routers from my iPhone and iPad with one of these cables.