Nikon D3100 video and viewfinder?

Is it possible to look through the viewfinder while recording video or must you always look on the LCD?

When in video mode, the camera's mirror is flipped up to expose the sensor the entire time. When the mirror is up, the viewfinder can't be used.

No. One more reason the dSLR was never meant to do video.

As with most DSLRs, the Nikon D3100 has an optical viewfinder that lets you view your subject through the lens. Unlike the D3000, the Nikon D3100 also offers Live View mode which allows you to preview the current subject on the LCD monitor. Video quality looks typical for its class, but with seemingly more rolling shutter artifacts than usual (and note that the lens VR makes a huge difference when shooting video).