See if you have an OSEPP Uno R3 Plus. If you do, then the Arduino 1.0.5 software package doesn't include the serial driver you need to talk to the board. When I plugged my board in all I got in my System Console was:
Sep 29 11:01:15 Tsukuyomi pcscd: Non-smartcard device launched pcscd [Vendor: 0X403, Product: 0X6001]
I found a form post at DroolFactory: Setting the Serial Port of a OSEPP Uno On Mac OS X Lion that explained how to fix it. Basically you download the Arduino 0023 package (the last version before Arduino 1.0.0) and install the FTDIUSBSerialDriver_10_4_10_5_10_6.pkg package, which installs serial drivers that support the FT232R chip that the OSEPP Uno uses. After that, the Arduino 1.0.5 software should work fine.