Isn't that true for all Raspberry Pi projects by the very nature of Raspberry Pis? They're just a different form factor and cost than other Linux machines, but otherwise are just a Linux box.
No. There are some that actually use the Raspberry Pi hardware, such as PiFM[0]. Others merely use GPIO, which of course can work on a number of other SBCs (eg a Beaglebone Black) or anything with GPIOs. The only Raspberry Pi specific thing here is how it connects to the webcam, because the Raspberry Pi doesn't expose the camera as a standard v4l device, they have their own way of doing it.
To a degree. Most Linux boxes don't have a couple dozen programmable IO pins and the other connectivity options for external sensors and such.
In the case of this project, you'd also have to do some modifications on the image-grabbing code. They're using a module that's specific to the Raspberry Pi camera module.
