I just tried powering my Raspberry Pi by connecting it to the USB port on my iMac and it worked just fine. My Pi setup is that it only has a USB wifi dongle and nothing else and runs in headless mode. Model B requires 5V at up to 700 mA. USB 2 on my iMac actually provides 5V at 500 mA. However, Apple has a note that it’s possible to power devices with higher current requirements, see this note. So I decided to give it a try and it worked. However, I didn’t see the RPi in the system information under USB on the iMac. Perhaps the USB is just acting as a power supplier, it doesn’t list it.
While it is good to know that the Raspberry Pi can be powered from an iMac, I am not going to really do this often. I will have it powered from a power outlet.