Do you know if they're using a Classic version of
Photoshop? (In other words, are they running
Photoshop under Classic mode?)

Version 5.5 - Yes, most likely in Classic mode.

I haven't heard of any problems with Photoshop in
particular, however using a given font in both Classic
and OS X is not always intuitive--but it's easy enough
to figure out.

In short, it is possible to activate a font in OS
X and have it NOT be active in Classic. You have to
think of these as two separate operating systems
(because they are).

Make sure your customers are activating the font
in both operating systems if they're using the font in
both Classic and OS X apps.

Hope this helps.

I have had this problem also. A PS type 1 font generated with Fog and installed on OS10.2.4 appears in TextEdit but not at all in Illustrator or Photoshop (v7 OSX native). Anybody else had any experience of this issue?

Looks like a few people have had problems:
http://www.myfonts.com/FontForums/read.php?f=6&i=41&t=41