Thanks Sensorjunkie for taking the time to put this all in one place! In the past few months I've learned a few things that might be helpful here.
First, most Melexis Thermopiles for sale have a wide field of view, about 35-degrees. There is a model F with a 10-degree field of view but seems difficult to get.
For reference, a standard webcam has a field of view of 40-degrees by 30 degrees. It's difficult to focus a thermopile because glass and plastic absorb around half of radiation between 3 and 20 micrometres. Germanium is the best material, but any research will show you that those lenses are a fortune. To get around the absorption issues in plastic you'll notice that the IR thermometer you buy from Sears uses a fresnal lens. These lenses magnify with the least amount of lensing material.
Also, let me point out a common misconception I come across, the infrared that the Melexis picks up has almost NOTHING in common with the infrared that your digital camera can be converted to see. Infrared photography deals with 800 to 1,000 nanometers, or just below the red spectrum. If you measure that distance as a football field, thermal radiation starts about 10 football fields lower. Silicon is basically transparent to that radition. YOU CANNOT MODIFY YOUR DIGITAL CAMERA OR BUY A LOW COST SECURITY CAMERA that sees true thermal radiation from your PC power supply, let's say.
In the January, 2010 issue of Nuts and Volts there is an interesting article by L. Paul Verhage, that uses a parallax thermopile for a near space infrared telescope. The Melexis could be used in similar applications. I think his technique for limited the field of view of the thermopile would work well with the Melexis.
Thanks again Sensorjunkie for making a go-to-place for getting the Melexis to work with Arduino!