This is just a prototype, but I think they're on to something. This little bugger picks up your phone, via Bluetooth, and interrogates the caller. If the caller is on a whitelist and they don't seem too agitated—the squirrel, apparently, can read voice patterns—this little fella will wiggle around to get your attention.
He's only at the MIT Media Lab right now, but expect something like him to appear very soon.
Cellular Squirrel, a robot that screens your calls [MobileBurn]