The way light and shadows fall can help create a 3D effect. And its particularly so if this effect is complemented by having suitable out of focus areas falling in the right areas. If the right photo comes along and the lighting does not produce a suitably 3D effect when I think this would help the image, I am not averse to doing a little vignetting in Photoshop to help it a long. But its all relative - photography is a 2D medium and 3D can really only be suggested - never achieved strongly.
This image for example I think has quite a 3D feeling because of the way the shadows fall helped a bit by where the point of focus is. You dont necessarily need a lot of OOF blur - enough to suggest depth may be all you need in some cases. In this first shot the OOF is very subtle but still helps.
_DSC4013a1 by
yoyomaoz, on Flickr
Similarly perhaps with this shot - I was experimenting with a Voightlander 50mm Nokton and its shallow depth of field. Here the DOF certainly has helped as has the lighting to some extent:
L1042013 by
yoyomaoz, on Flickr
Here is another shot where lighting and point of focus combine to help produce a 3D effect:
_DSC5552 -1 by
yoyomaoz, on Flickr
As it does here where the out of focus areas are even stronger
L1041851a by
yoyomaoz, on Flickr
And finally lets not forget images which suggest a 3D effect because of perspective (the oldest trick in the artist's book!)
_DSC5430Aa by
yoyomaoz, on Flickr