Digital versus film really doesn't have any bearing on how the depth of field looks in the manner you are talking about--in fact, most digitally shot movie use the same lenses as film productions. A shallow depth of field image captured by a high-end digital still camera has pretty identical image characteristics as with a film camera, in this respect. I think its more about the way the human eye sees things versus the way a lens does. Part of it may be the stereoscopic effect of the human eye (if you look at something up close while covering one eye, the "blurryness" of the BG seems more pronounced IMO, or perhaps you become more aware of it), but I think its really just the way glass lenses capture images. They use the same principles as the human eye but obviously rods and cones and soft tissue is not the same as glass and mirrors and emulsions.