Check out this awesome PIR Motion Sensor, complete with infrared goodness!
PIR sensors are a very proven technology - they're in practically every security system you've seen, so they've got to be pretty good. Now Parallax has taken these useful sensors, added their usual brilliant twist to them, and now make them available to you in a tidy, efficient package!
Moving blobs of IR heat energy (that'd be me, you, and the dog) are detected up to 30 feet away by the built-in Fresnel lens. A simple 4-pin (ground, power, signal, enable) output is compatible with practically any microcontroller.