I am on Mac OS X 10.7 (Lion) as well.
So, when I hooked up a second Arduino board I bought yesterday, an OSEPP Uno Rev 1.1, I could not load sketches onto it.
I went to the getting started website for the OSEPP Uno, which suggested setting the board to an “Arduino Duemilanove or Nano w/ ATmega328″ to get it to work. Still nothing.
I looked at the Serial Ports but there were no usbserial ports listed. So, start debugging. I opened the Console and tried unplugging/plugging in the board, sure enough there was an error:
kernel: 0 0 AppleUSBCDC: start - initDevice failed
So, rereading the getting started site, I released that since this board was a little older I might just need to the older drivers.
I downloaded the Arduino 0023 drivers from the Arduino Software page for Mac OS X. I installed just the FTDIUSBSerialDriver_10_4_10_5_10_6.mpkg file.
Restart the Arduino IDE and voila, there should not be a /dev/tty.usbserial-* device that will be the OSEPP Uno board. You can then load your programs.
Hope that helps.