Drobo + Time Capsule
I've plugged in a Drobo S into the USB port on an Airport Extreme when I needed fast and dirty file sharing. It worked fine. It was only for a week, but I didn't notice any sort of unusual behavior. 

Once you've done the initial setup/config via the Drobo interface, the drive acts just as standard USB drive. It's not the fastest thing in the world, but you're limited by the USB 2 anyways, so your experience regarding speed will depend more on your actual usage.