Serial Port Already In Use - Unable To Do Anything With My UNO

Hi all, I've just got my first Arduino board, the UNO, and I'm trying to get it running with my iMac (Lion) and struggling! When i plug it in I don't get the network popup that I believe I am supposed to see, however the power LED on my board is lit and the 'L' LED is blinking. I have selected my board in the Arduino app and the serial port as /dev/tty.usbmodem1d11, however when I go to upload a sketch I get the error message:

Serial port '/dev/tty.usbmodem1d11' already in use. Try quitting any programs that may be using it.

I can't see why any programs would be using it as I'm trying this fresh from a restart. As I said i'm completely new to this and trawling various forums hasn't solved the problem so any help at all would be appreciated!

Thanks!

BTW if I can provide any more info to help diagnose the problem just let me know!