You can do it with a quad mesh with an alpha-d texture positioned (parented to) the camera, like others here have said. Filax has done this in some of his demos - I tried to recreate it and managed it fairly easily.
Make sure the UV texture scale is low enough to create a large number of lines over the quad (ie. over 1600 texture lines on the quad) so that it looks correct at several resolutions.
If you test zooming the position of the quad away from and towards the camera, you should be able to position it perfectly to get a good looking scan line effect.
Make sure the UV texture scale is low enough to create a large number of lines over the quad (ie. over 1600 texture lines on the quad) so that it looks correct at several resolutions.
If you test zooming the position of the quad away from and towards the camera, you should be able to position it perfectly to get a good looking scan line effect.