Here are a couple of additional suggestions, but after these I unfortunately have run out of suggestions for you.
Are you using one of the current Apple USB or wireless Apple keyboards? If you are using an older Apple or a 3rd party keyboard then it's possible your system is not recognizing the key presses required to run AHT.
Another way to verify the USB ports on your iMac is to use the built in 'System Profiler' feature of Mac OS X. First, plug in a USB device to the suspect USB port, then pull down the Apple menu and select 'About this Mac'. When the 'About this Mac' window opens click on the 'More Info...' button, then click on the 'System Report...' button. Once the system profiler window open click on 'USB' in the Hardware column on the left side to display your USB device tree. You should see a list of all the USB devices that are attached to your system including the FaceTime camera, SD card reader, and your Bluetooth card that are inside your iMac. Do you see the device attached into the suspect USB port? If you move the device to another USB port you can press the command-R keys to refresh the device tree list. If the known good device doesn't show up when it is connected to the suspect port then you have confirmed that you have an issue with that port. Even if you choose not to take your system in for service I still recommend contacting Apple so that you can create a case to document the issue in case you want to fix it at a later date.