I'm pretty sure that the cameras in Microsoft Surface solutions don't use cameras that see the heat from your finger, just IR light around the 800-950nm range. After all, the systems are heavily pitched with their object recognition based on the IR tags that are affixed to devices, and the tags (stickers) don't put out heat but are easily capable of reflecting IR light from below. I believe it is a DI (diffused illumination setup) with the 5 cameras, where there is an IR source in the enclosure, and the cameras have color filters to only allow IR light. Actually it is not all that difficult to build a similar system. Nearly 3 years ago I had a simple multi-touch system running at home that was quite similar to the technology behind Surface, but I only used 1 camera instead of 5.