Well, this isn't as strange as it may first sound. For example,
photographs which are slightly blurred (eg because of camera motion)
may look perfectly ok when printed at 10x15cm, but if printed at
60x90cm they won't. It's the same idea.

Or think of looking at a slide by holding it up against a light as
opposed to projecting it onto a screen (or, better, printing it at some
large size, say 120x180cm for shock value). In the second case, isn't
it true that various imperfections (such as being slightly out of
focus) become more obvious? Again, same idea. Well, at 120x180cm it'll
be rubbish anyway (if 135mm film), but never mind.

As an off-topic aside, some time ago I happened to be in Amsterdam and
went to an exhibition of award-winning journalistic photographs. It is
quite spectacular how irrelevant strong chroma noise and low resolution
become when looking at a (probably 8mp) 60x90cm print showing some poor
guy without arms being helped to get dressed by his 5 year old son (and
this in a refugee camp). Puts everything into proper perspective.

For the purposes of DOF evaluation in the print the viewing distance is
typically quite close (about twice the diagonal).  Otherwise it's like
printing small, giving the appearance of more DOF.

Cheers,
Alan

PS: you don't need to digitally sign your posts here.

Many things are factors. The type of surface on which the print is
made. How much it was sharpened. Whether there is anything at distances
between the subject and the background. (Try printing the same thing on
glossy and matte papers; also sharpen the one on the glossy paper more,
and you'll see what I mean). It depends on who is viewing, and which
(if any) pair of glasses he is wearing. It depends on the lighting. It
depends on how much he's had to drink. Where he is doing the viewing
(in a gallery? a moving car? a crashing airplane? his ability to
discern fine points, as well as his interest in doing so, will be
different in each case).

However, when we speak of DOF, there is a widely accepted definition,
given here
http://en.wikipedia.org/wiki/Depth_of_field
(look at the equations, not the blah blah). So when DOF is discussed,
either we talk about this thing (in which the viewing distance, sensor
size and print size enter indirectly and together through the circle of
confusion), or we factor in other things, in which case we are talking
about something else. Nothing wrong with that, of course: it is obvious
that if I am in a pitch black room, the formulae given in the link do
not produce the correct DOF (as I cannot see the print at all).

Some things, however, are implicitly assumed in such discussions;
repeating them again and again, and in general providing a complete
self-contained context around each sentence (such as specifying that
DOF does not mean some random concept that could have this name but the
particular concept that is widely accepted) gets tiring.

To reiterate: In the definition of DOF that I thought was being
discussed, the viewing distance enters through the circle of confusion.
So does the printing size, and the sensor size (which also affects the
focal length if you want to equalise fields of view). In other
definitions, other things happen.