No usb ports are under Tools > Serial Port > ... A few days ago it was working fine, and then it stopped working on both mac os 10.4 and arch linux at the same time (dual boot). Nothing on the troubleshooting page helps. Although I haven't tried reinstalling the bootloader.
I have an arduino decimila, macbook 2g, and arduino ide 0012.
if already plugged in to the computer directly, try a different port.
power down and unplug everything (computer, external hub, etc). boot up and try again.
if on a Mac, look at the system profiler (apple menu -> about this mac -> more info) and look at the usb info. If it shows up here, it might mean that something has puked on the drivers.